Make Annotation Layer Names Unique in DB

[fm3]

The Front-end already enforces unique url-safe layer names as of #6289 – this PR adds the same constraints in the back-end.
Note that the evolution is not fully automatic this time (would have become complex). Instead, instructions are added to the migration guide on how to manually migrate the db content.

Some backend changes followed from the name now being non-optional.

Additionally, The layer name displayed by the front-end for Skeleton Annotation Layers is changed from »Skeletons« to »Skeleton« as discussed here – Note that in the Skeleton case, the front-end does not use or set the annotation layer name from the DB, but instead uses this string. This is a special case enabled by the fact that there is currently always at most one skeleton layer.

Steps to test:

• evolutions should be equal to schema (asserted by CI)
• users should be able to set volume layer names according to the constraints

Issues:

• fixes Make annotation layer names unique per annotation #6251

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• Updated (unreleased) migration guide if applicable
• Ready for review

[jstriebel]

LGTM 👍