Resolve "Finally fix lessons_on_day property"

Closes #145

See merge request !183

# Conflicts:
#   CHANGELOG.rst

Resolve "Preference section is called Chronos"

Closes #144

See merge request !180

Fixup changelog chaos and set version to 2.0b2

See merge request !179

(cherry picked from commit 3f4ce4a7)

Resolve "Show abbreviations in the substitution plan only once"

Closes #140

See merge request !178

Resolve "Migrations broken in 2.0b0"

Closes #142

See merge request !176

Resolve "Unique constraints doesn't include all necessary fields"

Closes #141

See merge request !175

Resolve "Keep a changelog header missing"

Closes #143

See merge request !177

Together with 18544e06, updating the existing
migration is safe, because

 a) if the migration failed somewhere due to the multiple leaf nodes, it was
    never applied, and changing it is safe;
 b) if it succeeded, it will be renamed to 0008_ later, and thus
    re-applied

Update poetry.lock with 2.0b0

See merge request !171

Resolve "Update feature list"

Closes #139

See merge request !170

Resolve "Fields must be unique per site"

Closes #137

See merge request !168

Prepare release 2.0b0

See merge request !169

Resolve "Fix migrations"

Closes #138

See merge request !167

Write changelog for 2.0b0

See merge request !166

Translations update from Weblate

See merge request !165

Currently translated at 100.0% (128 of 128 strings)

Translation: AlekSIS/AlekSIS-App-Chronos
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis-app-chronos/de/

Update translation files

See merge request !164