Issue filters are being used on repo list page and on milestone issues
page, and the code is mostly duplicated.
This PR does the following changes:
- move issue filters into a shared template
- allow filtering milestone issues by project, so no need to hide this
filter on milestone issues page
- remove some dead code (e. g. issue actions in milestone issues
template)
- fix label filter dropdown width
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Co-authored-by: 6543 <6543@obermui.de>
