Add 'user_approval' submodule

This PR adds a new submodule feature 'usre_approval'.
Using this submodule, you can require new registrants to be approved by
existing users (especially by an administrator team, I suppose).

Although Sorcery already has a feature named 'user_activation' and the
submodule has very similar function to 'user_approval',
'user_activation' intends to require new registrants to activate their
accounts themselves.
As contrasted, 'user_approval' intends to enable the admin team to
decide whether they accept the new registrant at the website.