Allow customizing notification templates per project (Netflix#3369)

* project specific templates

docs/docs/administration/settings/messaging.mdx

@@ -0,0 +1,50 @@
+# Messaging
+
+Dispatch supports sending email notifications to participants of, for example, an incident.
+
+## Notification templates
+
+Templates for emails are [part](https://github.com/Netflix/dispatch/tree/master/src/dispatch/messaging/email/templates) of Dispatch
+and are [Jinja](https://jinja.palletsprojects.com/) templates that during runtime are compiled into [MJML](https://mjml.io/) format.
+
+There is a way to customize these templates. To do this, if you run Dispatch with [Docker Compose](https://github.com/Netflix/dispatch-docker/),
+mount a volume with a customized templates dir as part of the docker compose:
+
+```
+  web:
+    image: dispatch-local
+    ...
+    volumes:
+      - "../dispatch-templates/messaging-email-templates:/usr/local/lib/python3.11/site-packages/dispatch/messaging/email/templates"
+```
+
+Such approach allows you to customize the common template for *all projects*.
+
+You can also "patch" the templates *per project*. Create a folder per project (identified by project id):
+
+```
+dispatch/messaging/email/templates/project_id/<project_id>/base.mjml
+```
+
+This will be used at the first place if exists,
+otherwise the resolution process will gracefully fall back to the default template:
+
+```
+dispatch/messaging/email/templates/base.mjml
+```
+
+
+## Markdown in the notifications
+
+:::warning
+Watch out for security implications related to unescaped HTML that may propagate through the system.
+:::
+
+By default, notification text is just a plain text with special characters and HTML escaped.
+
+It is possible, however, to enable Markdown syntax with a server setting:
+
+```
+DISPATCH_MARKDOWN_IN_INCIDENT_DESC=True
+DISPATCH_ESCAPE_HTML=False
+```

src/dispatch/messaging/email/utils.py

@@ -3,6 +3,7 @@
 import subprocess
 import tempfile
 
+import jinja2.exceptions
 from dispatch.config import MJML_PATH
 
 
@@ -20,7 +21,7 @@
 log = logging.getLogger(__name__)
 
 
-def get_template(message_type: MessageType):
+def get_template(message_type: MessageType, project_id: int):
     """Fetches the correct template based on the message type."""
     template_map = {
         MessageType.incident_executive_report: ("executive_report.mjml", None),
@@ -45,19 +46,27 @@ def get_template(message_type: MessageType):
         ),
     }
 
-    template_path, description = template_map.get(message_type, (None, None))
+    template_key, description = template_map.get(message_type, (None, None))
 
-    if not template_path:
+    if not template_key:
         raise Exception(f"Unable to determine template. MessageType: {message_type}")
 
-    return env.get_template(os.path.join("templates", template_path)), description
+    try:
+        template_path = os.path.join("templates", "project_id", f"{project_id}", template_key)
+        template = env.get_template(template_path)
+    except jinja2.exceptions.TemplateNotFound:
+        template_path = os.path.join("templates", template_key)
+        template = env.get_template(template_path)
+    log.debug("Resolved template path: %s", template_path)
+
+    return template, description
 
 
 def create_multi_message_body(
-    message_template: dict, message_type: MessageType, items: list, **kwargs
+    message_template: dict, message_type: MessageType, items: list, project_id: int, **kwargs
 ):
     """Creates a multi message message body based on message type."""
-    template, description = get_template(message_type)
+    template, description = get_template(message_type, project_id)
 
     master_map = []
     for item in items:
@@ -67,9 +76,11 @@ def create_multi_message_body(
     return render_html(template.render(**kwargs))
 
 
-def create_message_body(message_template: dict, message_type: MessageType, **kwargs):
+def create_message_body(
+    message_template: dict, message_type: MessageType, project_id: int, **kwargs
+):
     """Creates the correct message body based on message type."""
-    template, description = get_template(message_type)
+    template, description = get_template(message_type, project_id)
 
     items_grouped_rendered = []
     if kwargs.get("items_grouped"):

src/dispatch/plugins/dispatch_google/gmail/plugin.py

@@ -103,10 +103,12 @@ def send(
             cc = kwargs["cc"]
 
         if not items:
-            message_body = create_message_body(notification_template, notification_type, **kwargs)
+            message_body = create_message_body(
+                notification_template, notification_type, self.project_id, **kwargs
+            )
         else:
             message_body = create_multi_message_body(
-                notification_template, notification_type, items, **kwargs
+                notification_template, notification_type, items, self.project_id, **kwargs
             )
 
         html_message = create_html_message(