Fix wrong image size in the partial manage/disabled_space

Was trying to render an invalid image and triggering a 500 error in production.
See more at #1082.

refs #1082
Fixed the problem when a user/space is created with the same identifier of another

When a user or a space are created the system try to create his room using the username/permalink like param. This can result in a error, because the room can't be created with a param equal to another. To resolve this issue a validate function was created in user/space model to verify if doesn't exist another model with the same identifier. If another identifier is found the model aren't created.

Fixed #1071
Adjustments and more tests after PR #9

Main changes:
* Validating the permalink and username on update too, since we don't properly
  block the user from changing these values yet.
* Brought back the check for errors on the bigbluebutton_room when validating
  a space. This should be reviewed, improved, and done for User too.
* Doesn't auto-suggest usernames on 'create' anymore, only on 'new', otherwise
  we might be overriding a value set by the user.
* Improved the js method to generate slugs more similar to slugs valid for
  usernames and permalinks.

refs #1071
Merge remote-tracking branch 'upstream/branch-v2' into branch-v2

Conflicts:
	app/assets/javascripts/app/registrations/_signup_form.js.coffee
	app/models/user.rb