Update GitHub template to steer feature requests to community forum

Thanks to @EwoutH for doing this on Signal-iOS and Signal-Android!

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,82 @@
+---
+name: 🛠️ Bug report
+about: Create a report about a technical issue
+title: ''
+labels: ''
+assignees: ''
+---
+
+<!--
+Our bug tracker is ONLY for bugs. It is not for feature requests, questions, or comments.
+
+Please fill out this template with all the information you have. We can't do much without
+both the logs and a detailed description of what you've encountered. Please do your best!
+
+Please note that this tracker is only for bugs. Please try these locations if you have a question or comment:
+
+  https://community.signalusers.org/
+  http://support.signal.org/
+  support@signal.org
+
+Lastly, be sure to preview your issue before saving. Thanks!
+-->
+
+- [ ] I have searched open and closed issues for duplicates
+<!--
+  You can search all issues here:
+    https://github.com/signalapp/Signal-Desktop/issues?utf8=%E2%9C%93&q=is%3Aissue
+  Replace [ ] with [X] once you've searched
+-->
+
+---
+
+### Bug Description
+
+<!-- Give an overall summary of the issue. -->
+
+### Steps to Reproduce
+
+<!-- Using bullet points, list the steps that reproduce the bug. -->
+
+1.  step one
+2.  step two
+3.  step three
+
+Actual Result:
+
+<!-- Describe the details of the buggy behaviour. -->
+
+Expected Result:
+
+<!-- Describe in detail what the correct behavior should be. -->
+
+### Screenshots
+
+<!--
+How to take screenshots on all OSes: https://www.take-a-screenshot.org/
+You can drag and drop images into this text box.
+-->
+
+### Platform Info
+
+Signal Version:
+
+<!-- You can see Signal's version number at Help -> About or File -> About Signal Desktop -->
+
+Operating System:
+
+<!-- Instructions for finding your OS version are here: http://whatsmyos.com/ -->
+
+Linked Device Version:
+
+<!-- Android: Settings -> Advanced,  iOS: Settings -> General -> About -->
+
+### Link to Debug Log
+
+<!--
+Immediately after the bug has happened, submit a debug log via View -> Debug Log, then copy that URL here.
+
+In most cases, a log from your other devices is also useful:
+  Android: https://support.signal.org/hc/en-us/articles/360007318591#android_debug
+  iOS: https://support.signal.org/hc/en-us/articles/360007318591#ios_debug
+-->