Resolve "Ignore incomplete substitutions feature is incomplete"

Closes #42

See merge request !123

Release 2.1.2

See merge request !121

Translations update from Weblate

See merge request !116

Currently translated at 100.0% (26 of 26 strings)

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

Update translation files

See merge request !122

Resolve "Option to hide incomplete substitutions from the plan"

Closes #39

See merge request !119

Prepare release 2.1.1

See merge request !118

Resolve "Allow selecting plan version and default to newest version"

Closes #40

See merge request !117

[Docs] Enable build

See merge request !115

Prepare release 2.1

See merge request !114

[Docs] Fix intersphinx mapping

See merge request !113

Translations update from Weblate

See merge request !112

Currently translated at 92.0% (23 of 25 strings)

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

Update translation files

See merge request !111

[Docs] Add references

See merge request !110

Add docs

See merge request !109