Upgrades (bootstrap, fontawesome, django, js dependencies) and static file compression

• Bump django-bootstrap to version 5 (Implements #158 (closed))
  • Exchange bootstrap files (js & css)
  • Adapt settings
  • Migrate to new version/resolve breaking changes
  • Move layout adaptions previously included in bootstrap.css into custom.css
• Upgrade to fontawesome 6
  • Use official fontawesome django app for static file handling
  • Switch to fork/new version of django-bootstrap (for icon field and rendering shortcuts)
  • Add migration to bump icon field of dashboard buttons to new version
  • Update templates to load new app and use new tags
• Upgrade django and other python dependencies (Implements #157 (closed))
  • Upgrade django to 4.1
  • Upgrade django-timezone-field, django-simple-history, django-debug-toolbar and django-bootstrap-datepicker-plus
  • Upgrade mysqlclient
  • Remove obsolete pytz
  • Adapt timezone handling code to new interface of TimezoneInfo (part of python since 3.9 and backport included in 3.8)
  • Drop support for python 3.7 (since Django 4 requires >= 3.8)
• Upgrade JS Libraries
  • Upgrade Fullcalendar to v6 (implements #186 (closed))
  • Upgrade jQuery to v3.6 (implements #185 (closed))
  • Upgrade Select2 to v4.0.13
  • Remove tag information leftover in track manager
  • Fix fullcalendar theming issues
• Add further tests to check sending of forms with little overhead
• Introduce static file compression
  • Add compressor and sass compiler dependencies
  • Introduce corresponding settings
  • Use compressor in templates
  • Move only partially used dependencies to higher level for better re-usability
  • Add custom tags to create css and js for fontawesome separately
  • This implements the first part of #162 (closed)
• Use sass compiling for bootstrap
  • Remove static version of bootstrap css
  • Add bootstrap sources
  • Add bootswatch sources and variable file
  • Improve the way admin css is handled
  • This finishes #162 (closed)