Allow scheduling notifications by setting a send datetime

See merge request !983

# Conflicts:
#   CHANGELOG.rst
#   aleksis/core/models.py

[Docs] Add missing bracket in config example

See merge request !984

Implement StaticContentWidget

Closes #662

See merge request !981

[Docs] Fix references

See merge request !982

Resolve "Add configurable username validator for signup"

Closes #664

See merge request !980

Update dependency django-phonenumber-field to v6

See merge request !774

Update dependency django-reversion to v5

See merge request !977

Fix typo

See merge request !979

[Docs] Fix redis address scheme

See merge request !978

Resolve "[AdditionalField] Allow setting `required` and `help_text`"

Closes #660

See merge request !976

Resolve "Feature: Account Menu in Top Navbar"

Closes #531

See merge request !907

# Conflicts:
#   CHANGELOG.rst

Resolve "Improve mobile design of login page"

Closes #572

See merge request !905

# Conflicts:
#   CHANGELOG.rst

Add retry button to 500 page

See merge request !972

Release 2.7.4

See merge request !969

Translations update from Weblate

See merge request !947

Currently translated at 100.0% (651 of 651 strings)

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

Update trans files

See merge request !968