Remove the MainUserId selecting radio button

It clutters the UI a lot for a relatively minor feature that very
likely is rarely used. Main user IDs don't change often and when they
do, they can simply be changed in the first user ID entry.

In future it might be nice to allow reordering of the user IDs
with drag and drop, but that really doesn't seem like a necessary
feature right now.
Align key info in KeyEditor and adjust key ID display
Make emails optional in user IDs

This is allowed in the RFC and can be desired if a key shouldn't
be bound to an email at all.