Add engineering proposal template for large changes

.github/ISSUE_TEMPLATE/proposal.md

@@ -0,0 +1,39 @@
+---
+name: Technical proposal
+about: propose a major technical or architectural change
+
+---
+
+<!--Use this template to help kickstart discussions of specific technical and architectural proposals. It's OK to be unclear about who should be involved or even what to propose to solve the problem under discussion.-->
+
+## Proposal:
+
+### Affected components
+<!--Which system components are (probably) in scope? (list as many as make sense)
+    - SecureDrop Server
+    - SecureDrop Workstation
+    - SecureDrop Client
+    - Don't know
+    -->
+
+### People and roles
+<!--Tag people who should be involved in this proposal.-->
+
+### Problem Statement
+<!--Describe the problem that needs solving, or the architectural change that you'd like to see.-->
+
+### Solution impact
+<!--If we do solve this problem, what impact does it have, and for who?-->
+
+### Requirements and constraints
+<!--What requirements would a solution to the problem above have to satisfy? What existing technical constraints would affect potential solutions?-->
+
+### Exploration
+<!--Provide more context for the problem and potential solutions. (OK to leave blank if you want more group input.)-->
+
+### Initial proposal
+<!-- If you have one, propose a solution to the problem. (OK to leave blank, OK to duplicate this section in future edits with alternative proposals.) -->
+
+### Selected proposal
+<!--Leave blank for now - this section will record the agreed-upon solution to the problem.-->
+