[WIP] Refactor volume undo mechanism

[philippotto]

• Adds BucketSnapshot class which is responsible for compressing/decompressing local/remote bucket data.
• Using BucketSnapshot, the undo saga could be slimmed down quite a bit (in code size as well as async complexities).

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• abc

TODOs:

• ...

Issues:

• fixes #

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(Please delete unneeded items, merge only when none are left open)

• Updated changelog
• Updated migration guide if applicable
• Updated documentation if applicable
• Adapted wk-libs python client if relevant API parts change
• Removed dev-only changes like prints and application.conf edits
• Considered common edge cases
• Needs datastore update after deployment