Remove some of the Tildes Groups interface

[zorbathut]

We're likely going to have only one group for quite a while. Remove the confusing bits of the interface.

New users should be auto-subscribed to that group without having to click anything; in fact, the entire subscribe/unsubscribe interface should be disabled for now.

[GutsyCrown]

The quick-and-dirty version of this should be done in #39 . New users should already auto-subscribe to all groups, so we just need to ensure that ~themotte is the only group in prod. If we decide to stick to 1 group permanently that should involve more fundamental changes.

[zorbathut]

Looks like this is done! :D

[zorbathut]

Reopening this for a few more issues:

• The themotte group is only created in dev mode, not in release mode. I'm not sure where this code should go, but it should go somewhere; we also need to be careful to make this play properly with Alembic.
• I suspect that new users aren't added to the group, only the dev user. Haven't checked this out yet, but I believe so. We'd probably need to add an autosubscribe in the new user functionality.

[zorbathut]

Thanks to gogmagog's research (I have no idea what their github name is), it looks like users are added to all groups, as per https://github.com/themotte/tildes/blob/master/tildes/tildes/views/register.py#L94 so maybe that's not an issue after all :)

But creating ~themotte is still an outstanding issue.