[AIRFLOW-3067] Display www_rbac Flask flash msg properly (apache#3903)

The Flask flash messages are not displayed properly.

When we don't give a category for a flash message, defautl
value will be 'message'. In some cases, we specify 'error'
category.

Using Flask-AppBuilder, the flash message will be given
a CSS class 'alert-[category]'. But We don't have
'alert-message' or 'alert-error' in the current
'bootstrap-theme.css' file.

This makes the the flash messages in www_rbac UI come with
no background color.

This commit addresses this issue by adding 'alert-message'
(using specs of existing CSS class 'alert-info') and
'alert-error' (using specs of existing CSS class 'alert-danger')
into 'bootstrap-theme.css'.

airflow/www_rbac/static/css/bootstrap-theme.css

@@ -4949,6 +4949,28 @@ a.thumbnail.active {
 .alert-danger .alert-link {
   color: #843534;
 }
+.alert-message {
+  background-color: #d9edf7;
+  border-color: #bce8f1;
+  color: #31708f;
+}
+.alert-message hr {
+  border-top-color: #a6e1ec;
+}
+.alert-message .alert-link {
+  color: #245269;
+}
+.alert-error {
+  background-color: #f2dede;
+  border-color: #ebccd1;
+  color: #a94442;
+}
+.alert-error hr {
+  border-top-color: #e4b9c0;
+}
+.alert-error .alert-link {
+  color: #843534;
+}
 @-webkit-keyframes progress-bar-stripes {
   from {
     background-position: 40px 0;

airflow/www_rbac/views.py

@@ -782,11 +782,12 @@ def delete(self):
         try:
             delete_dag.delete_dag(dag_id)
         except DagNotFound:
-            flash("DAG with id {} not found. Cannot delete".format(dag_id))
+            flash("DAG with id {} not found. Cannot delete".format(dag_id), 'error')
             return redirect(request.referrer)
         except DagFileExists:
             flash("Dag id {} is still in DagBag. "
-                  "Remove the DAG file first.".format(dag_id))
+                  "Remove the DAG file first.".format(dag_id),
+                  'error')
             return redirect(request.referrer)
 
         flash("Deleting DAG with id {}. May take a couple minutes to fully"
@@ -2097,7 +2098,7 @@ def varimport(self):
             else:
                 d = json.loads(out)
         except Exception:
-            flash("Missing file or syntax error.")
+            flash("Missing file or syntax error.", 'error')
         else:
             suc_count = fail_count = 0
             for k, v in d.items():
@@ -2108,7 +2109,7 @@ def varimport(self):
                     fail_count += 1
                 else:
                     suc_count += 1
-            flash("{} variable(s) successfully updated.".format(suc_count), 'info')
+            flash("{} variable(s) successfully updated.".format(suc_count))
             if fail_count:
                 flash("{} variables(s) failed to be updated.".format(fail_count), 'error')
             self.update_redirect()
@@ -2198,10 +2199,9 @@ def action_set_running(self, drs, session=None):
                 dr.state = State.RUNNING
             models.DagStat.update(dirty_ids, session=session)
             session.commit()
-            flash(
-                "{count} dag runs were set to running".format(**locals()))
+            flash("{count} dag runs were set to running".format(**locals()))
         except Exception as ex:
-            flash(str(ex))
+            flash(str(ex), 'error')
             flash('Failed to set state', 'error')
         return redirect(self.route_base + '/list')