Django-jazzmin

Hotfix for search filters

- Allow customisation of buttons
- Continued improvement and tweaks on dark themes
- Upgrade to latest adminLTE (which includes more dark mode support)
- Minor bugfixes and translations
- Drop support for python 3.5, start testing against 3.9
- Add black and mypy checks in

Fixes a reference to an old static file

**Themes:**
- Add in theming support via UI customiser
- Move all library static files into vendor
- Adds in dropdown for choosing theme in UI customiser drawer
- Adds in all current bootswatch themes
- Adds theme-{theme_name} into body classes (for theme specific CSS rules)
- Disables modal popups for now by default (can be enabled), as they are broken on mobile
cleans up and groups sidebar customisations

**UI tweaks:**
- Whether to constrain the page to a box (leaving big margins at the side) with "layout_boxed": False,
- Make the footer sticky, keeping it in view all the time with "footer_fixed": False,
- Make the sidebar sticky, keeping it in view as you scroll with "sidebar_fixed": False,
- Make the top navbar sticky, keeping it in view as you scroll with "navbar_fixed": False,

**Locale support:**
- Add locale support for `hu` and `ru`

Minor bug fixes, tidying up code, small UI tweaks on inlines

- Compatibility with MPTT
- Compatibility with django-treebeard
- Compatibility with django-solo
- Compatibility with import_export
- convert all popups to modals See https://django-jazzmin.readthedocs.io/configuration/#related-modal (Big thanks to KarimTayie)
- Various minor bugfixes
- Rewrite test app
- Development docs/guidelines at https://django-jazzmin.readthedocs.io/bugs_and_features/

- Jazz up Stacked Inlines
- Added ability to sort models or custom links on side menu
- Fix minor bugs on Tabular Inlines
- Title languages list