Allow users to leave a workspace that isn't public and isn't owned by that user #431
Description
Applicable to any workspace in the 'Shared with me' list.
Allows users to leave workspaces that, for example,
- are no longer of interest to them;
- contain content that keeps coming up, unwanted, in their general searches;
- contain potential upsetting material;
- etc
This would be added to the semantic UI menu for an open workspace once implemented ( #585 )