Resolve "Delete UNTIS lessons without lesson periods while importing"

Closes #20

See merge request !58

Update poetry.lock

See merge request !59

Make Celery non-optional

See merge request !56

Add template for pull requests at GitHub for mirroring

See merge request !55

Update tox.ini; compile messages before building dist

See merge request !54

ugettext lazy is deprecated.

See merge request !51

Merge branch '18-untis-globalpermissions-aleksis-core-w002-model-globalpermissions-in-app-config-aleksis-apps' into 'master'

Resolve "untis.GlobalPermissions: (aleksis.core.W002) Model GlobalPermissions in app config aleksis.apps.untis doesnot derive from aleksis.core.mixins.ExtensibleModel"

Closes #18

See merge request !53

Fix testsuite

See merge request !52

Update tox.ini. Advances AlekSIS-Core#338

See merge request !50

Update tox.ini. Advances AlekSIS-Core#338

See merge request !49

Merge branch '15-substitutions-and-maybe-other-data-entries-aren-t-deleted-if-they-are-out-of-validity' into 'master'

Resolve "Substitutions (and maybe other data entries) aren't deleted if they are out of validity"

Closes #15

See merge request !48

Resolve "Filter by term while importing supervisions"

Closes #16

See merge request !47

Out of validity = Linked to a validity, but the validity range ends before the substitution's date

Close #15

Close #16

Currently translated at 100.0% (38 of 38 strings)

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

Resolve "Import supervisions being linked to a validity range"

Closes #12

See merge request !46

Resolve "Import absences correctly"

Closes #11

See merge request !45

Set comment in revision to attribute the deletion to the importer

See merge request !44

Resolve "Set teachers list to [] if there are no old teacher and no new teacher"

Closes #10

See merge request !42