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.

Advances #124

Add support for django-js-reverse

See merge request AlekSIS/AlekSIS!121

Make site title (used in title and nav header) configurable by constance settings

Closes #155

See merge request AlekSIS/AlekSIS!120

Close #155

Resolve "Redesign exception templates"

Closes #147

See merge request AlekSIS/AlekSIS!119

This reverts commit cae5ca3d.

Make constance configs usable in other settings

See merge request AlekSIS/AlekSIS!118

Introduces a utility function lazy_config() that can lazily bind
a constance config key. As an example, it is used to use the primary colour
configured through constance as the app's theme colour.

Advances #139

Resolve "Use or create external library for template mailer"

Closes #152

See merge request AlekSIS/AlekSIS!117