Groups fixes

[andyception]

Hey, I just tested our BitPoll instance and found some minor bugs concerning the Groups feature:

• dots "." don't work in group names, even though it specifically states they should work
• group names can't be changed
• groups can't be deleted
• The invitation link for groups contains a "https://" too much (the link sent is "https://https://****/groups/")

Shouldn't be too hard to fix these but it would increase the usability of the feature quite a bit. Tested with version 0c83e17

[Akasch]

The first and last point should be fixed now. The other two are more work.