Prepare release 2.0rc1

See merge request !77

(cherry picked from commit 68b2c778)

(cherry picked from commit 53e2eabd)

Update pyproject.toml, poetry.lock and tox.ini for 2.0b0

See merge request !74

Update translation files

See merge request !72

Update changelog for 2.0b0

See merge request !71

Remove XML importer for now

See merge request !70

This can be reverted completely when working on #7 outside of the release cycle.

Resolve "Rename rules to allow differentiation from permissions"

Closes #24

See merge request !69

Update poetry.lock

See merge request !68

Resolve "Extra lessons are imported without linked school terms"

Closes #23

See merge request !67

Denote Teckids e.V. as licensor as defined by EUPL

See merge request !66

This is important to clarify the jurisdiction and court location in case of
license disputes.

Update copyright information for 2021

See merge request !64

Update poetry.lock

See merge request !65

Update dependency defusedxml to ^0.7.0

See merge request !63

Resolve "Break supervisions must be deleted cleanly"

Closes #21

See merge request !62

Trigger dist pipeline on master

See merge request !61

Update ci config

See merge request !60

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