Allow configuration of new user verification method

Originally suggested in https://github.com/flarum/core/issues/641 by @moutonnoireu, who would like the ability to turn off email verification of new users. It might also be a good idea to consider other user approval methods at the same time. Common options are:
- Email verification
- Hold for admin approval
- Both of the above
- Nothing (automatic approval)

The second option would require a permission to determine who can approve registrations. Some forums may want to allow mods to approve new users; others may want to reserve this power for admins only.

Start by reading the original proposal in flarum/core#641 and compare it with the approval methods listed here. Define how email verification, admin approval, both, and automatic approval should be configured, including who may approve registrations. Done means the supported methods and required approval permission are specified and implemented consistently.

Written by the indexing model from the issue text.