Separate discussion forum from issue tracker
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## Summary
A new tab "Forum" is implemented next to "Issues". This forum hosts questions and answers sorted by topics.
## Background
It is very common for a popular project to receive questions filed as an issue to the issue tracker:
* How do I use ...?
* This does not work with XYZ. What should I troubleshoot...?
* I can't compile it. Am I doing something wrong?
* What do you think about integrating it with FooBar?
This increases the burden to the project maintainers and contributors during the triaging process. From the workflow of a contributor, their time is logically divided between two different activities (a) **triaging** defects, tasks, and merge requests and (b) providing **supports** (answering questions etc). A separate discussion forum would be extremely useful for this workflow.
Some benefits for the issue tracker:
* Searching the issue tracker does not result in tons of unrelated hits
* A question filed into the issue tracker can be moved to the forum
* Unrelated comments ("thread hijacking") can be spawn into its own discussion topic
Additional benefits for the discussion forum:
* Important discussion topics (FAQ, common troubles, known problems, ..) can be pinned for visibility
* A good new idea from a discussion can be spawn into a feature request (without the extra noise)
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