add check to prevent users from revoking their own access

[icewind1991]

When changing ACL rules, check if the new rule set would lead the current user without read permissions

• make the UI properly handle the error
• (maybe) add an option to force revoking the access anyway

[jancborchardt]

@icewind1991 since you said

Would it maybe make more sense to just fully prevent users from revoking their own access?

I would say the modal is better, because maybe you do want to get out of the group folder?

Access removal
Proceeding will remove your own access to this group folder.
[ Cancel ] [ Remove own access ]

[provokateurin]

Fixes #2822