Calendar events and iCal feeds

See merge request !1148

Update dependency graphene-django to >=3.0.0, <=3.1.3

See merge request !1294

Merge branch 'calendar-object-feeds' of edugit.org:AlekSIS/official/AlekSIS-Core into calendar-object-feeds

Resolve "Provide URL representation for every object"

Closes #871

See merge request !1268

Commit fa4df9e5 had unresolved merge
conflicts. They are merged now. This also adds a newline the
aleksolotl missed.

Resolve "'fields_for_model' is not defined in ExtensibleForm"

Closes #887

See merge request !1292

This allows, in the nFC context, for more privacy in the plain
data written to tags.

Update dependency graphene-django-cud to ^0.11.0

See merge request !1291

Resolve "Logging broken with format string"

Closes #888

See merge request !1293