This adds the ability to pin important Issues and Pull Requests. You can
also move pinned Issues around to change their Position. Resolves #2175.
## Screenshots
The Design was mostly copied from the Projects Board.
## Implementation
This uses a new `pin_order` Column in the `issue` table. If the value is
set to 0, the Issue is not pinned. If it's set to a bigger value, the
value is the Position. 1 means it's the first pinned Issue, 2 means it's
the second one etc. This is dived into Issues and Pull requests for each
Repo.
## TODO
- [x] You can currently pin as many Issues as you want. Maybe we should
add a Limit, which is configurable. GitHub uses 3, but I prefer 6, as
this is better for bigger Projects, but I'm open for suggestions.
- [x] Pin and Unpin events need to be added to the Issue history.
- [x] Tests
- [x] Migration
**The feature itself is currently fully working, so tester who may find
weird edge cases are very welcome!**
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Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>
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