Add frontend validation to "Restrict membership by email" field in Group -> Settings -> General -> Permissions, LFS, 2FA

The following discussion from !35543 (merged) should be addressed:

• @alexbuijs started a discussion: (+2 comments)

I was thinking about a frontend validation. Now we allow these entries:

• spaces
• non word characters
• no dots

Perhaps copying this really basic regex from the backend as a validation for a followup would make a lot of sense: /\w*\./.