Improve announcement queries

Necessary for AlekSIS-App-Chronos!41 (merged)

added part::backend label

mentioned in merge request AlekSIS-App-Chronos!41 (merged)

changed the description

For the unrelated changes, please clean up the original commits by rebasing.

changed milestone to %2.0a2

added 9 commits

• 47530bfd - Add extensible form which allows to add elements to django-material form layouts
• 3ee1a665 - Use ExtensibleForm for AnnouncementForm
• 3fa50753 - Add custom QuerySet for Announcement and move filter methods
• ad020641 - Add option to show recipients to common announcement template
• e91551da - Use ExtensibleForm from mixins.py
• 7abf8e9d - Fix use of datetime in announcement form
• 073a07d1 - Make filter methods for announcements more intuitive
• 42ad6ce2 - Add extra query method for announcements for a set of days
• ce2b6981 - Improve i18n for time interval

Compare with previous version

resolved all threads

added 7 commits

• ce2b6981...ac28e483 - 5 commits from branch master
• 855137e4 - Merge branch 'master' into feature/improve-announcement-queries
• 9e849897 - Rename queryset methods for announcements

Compare with previous version

merged

mentioned in commit e0094cb3