Resolve "Add helper function to filter a queryset by a rule"

Closes #293

See merge request AlekSIS/official/AlekSIS!318

Fix backup options

See merge request AlekSIS/official/AlekSIS!343

Fix translation source

See merge request AlekSIS/official/AlekSIS!344

# Conflicts:
#	aleksis/core/tasks.py

Fix add_class_to_el templatetag to work with lists for classes

See merge request AlekSIS/official/AlekSIS!342

Include email extension in makemessages called by dev.sh

See merge request AlekSIS/official/AlekSIS!340

Delay sending of notification until it's saved in database

See merge request AlekSIS/official/AlekSIS!341

Currently translated at 99.7% (362 of 363 strings)

Translation: AlekSIS/AlekSIS
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis/de/

Currently translated at 99.7% (362 of 363 strings)

Translation: AlekSIS/AlekSIS
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis/de/

Resolve "Support custom authentication backends"

Closes #295

See merge request AlekSIS/official/AlekSIS!334

Fix doubled dots in path_and_rename and add a test for it

See merge request AlekSIS/official/AlekSIS!337

Add some CSS classes for tables in printout views

See merge request AlekSIS/official/AlekSIS!338

As os.path.splitext returns the file extension
including a dot, adding an extra one is superfluous.

This reverts commit fd60f148.

Add a common class-based delete view (including template)

See merge request AlekSIS/official/AlekSIS!331

Update dependency isort to v5

See merge request AlekSIS/official/AlekSIS!330