Add option to require admins to accept new registrations

It allows admins to set a flag in the site that will require admins to accept
new registrations. The users are allowed to register new accounts, but they will
only be allowed to login once they are accepted.
By default this flag is off, so the site behaves as before: registration is open.

This was implemented in mconf-rnp/mconf-web, and this implmementation is based
on that, with a few more features.

refs #953
Merge remote-tracking branch 'upstream/branch-v2' into branch-v2

Had to make a few adjustments since now the "approve registrations" feature (#953)
is on mconf/mconf-web, so it could be removed from this fork.

Conflicts:
	app/controllers/users_controller.rb
	app/models/user.rb
	app/views/manage/_enabled_user.html.haml
	config/locales/en/devise.yml
	config/locales/pt-br/mconf.yml
	spec/factories/user.rb
	spec/models/user_spec.rb
Generate unapproved users on db:populate

refs #953
Few small layout fixes

- More consistent colors for notifications
- Design fixes in the frontpage
- Moving some strings to default places in the locales
- Changed the way input hints are shown to be less annoying
Merge remote-tracking branch 'upstream/branch-v2' into branch-v2

Conflicts:
	app/views/registrations/_signup_form.html.haml
	config/locales/pt-br/mconf.yml