Move locale field from WorkspaceMember to UserWorkspace

[FelixMalfait]

The preferred local field should be deleted on WorkspaceMember.
We should create a new one on UserWorkspace instead. Because we need to local information at a pretty high level / in a metadata or api context, and we don't want to have to go into the workspace schema for that.

[FelixMalfait]

OK we can wait for this, in the end I opted for a pattern where the locale is passed as a header to the API and not retrieved from the logged-in user, which makes this issue less urgent