The organization in the profile view is now the user institution

There's still a string Profile#organization, but it should not be used. Since
the user has an institution already set, the profile should show this institution
instead of Profile#organization.
The string was kept there but it's not used anymore.
Fancier approve button in the list of users

Using an icon instead of a text, more like the other options available in the
same view.
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
Suggest some institutions on a blank search refs #950
Added configurable limit for users in an institution #refs 955
Several enhancements to look and feel of institutions and management
Ensure users have an institution when registering refs #954
Correctly skip validation of admin in db.seed