ExtensibleModel is now an abstract base model.

Advances #141.

Add weekday field for constance

See merge request AlekSIS/AlekSIS!128

Bug fix for merge_app_settings

See merge request AlekSIS/AlekSIS!127

Update contribution guidelines

See merge request AlekSIS/AlekSIS!126

Without this bug fix the function doesn't import the settings module but the apps module.

Also, restructure a bit on the go.

Move path_and_rename to core_helpers.py and delete old helper.py

See merge request AlekSIS/AlekSIS!125

Load calendar labels from calendarweek library

See merge request AlekSIS/AlekSIS!122

# Conflicts:
#   pyproject.toml

Resolve "Background Tasks / Task Broker"

Closes #154

See merge request AlekSIS/AlekSIS!123

Update dependency html2text to v2020

See merge request AlekSIS/AlekSIS!124

This allows code that does not need Celery to also be run synchronously, e.g.
if the administrator does not want or cannot run Celery.

Setting `ALEKSIS_CELERY__EMAIL=true` on environment, or using the
equivalent in a config file, make all user mail sent by Django
asynchronous automatically without needing code changes. At the time of
writing, toggling this setting will e.g. make the notification sender
return immediately, and hand off the actual mail sending to Celery.