Add 'user_approval' submodule

[iTakeshi]

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.

[arnvald]

Hi @iTakeshi

thanks for your PR 💙
There's similar situation as in #535 - I'm not sure if this should be part of the core gem, or should be available as a plugin. I'm working on making Sorcery easy to extend by external gems and maybe it will be better to release it separately.
However, I'm not sure yet how it will work out, so I'm leaving this PR open and I'll let you know as soon as we decide about what to keep in the core and what to have as external plugins

[iTakeshi]

Hi @arnvald

👍 for your plan to introduce a pluggable architecture and I believe this make the gem more awesome!