Remove private message sent to institution admins when a user registered

We use emails now.
Fix ExceptionNotification after migration to 4.0.x (for Rails 4)

The way it is configured changed a lot, had to create the initializers again.
It also works for exceptions thrown in resque jobs :gem:

Added tests for the rescue_from blocks in ApplicationController.
Update bigbluebutton_rails to fix errors in a rake task

Same as 01a8c1f
Fix failing tests after fa1c6f70f64d6

Not entirely sure yet this is the best solution. Works just as it did
before, but changing the code so that it can be properly tested is not really
the best solution.

However, now specs are tested as in a development environment (throwing
exceptions instead of rendering error pages), while features are tested as
in a production environment (catching exceptions and rendering error pages).
This seems like a good improvement.
Fix invalid method calls when there's an error signing in via LDAP
Better validators for username and permalink uniqueness

It was working for most of the cases, except when we tried to create a user
or space with a username/permalink that would *not* conflict with other user
or space, but that would generate a room that would conflict with another
room.

Created validator classes to do proper validations over usernames, permalinks
and room params.

refs #609

Cherry-pick 52841ba
Conflicts:
	app/models/user.rb
Fix noty's close.png not being found in production