Force user to select an institution when registering

When register in site is available and a users try to register himself him should be forced to select an institution. To guarantee this, a verifying method was made inside the registrations controller. Making the verification this way ensure that administrators can continue creating users without institution.

refs #1352
Merge branch 'master' into Bug1352
Improve how we require an institution to be selected when registering

- Build the resource as devise does and set all possible errors on it
- Keep the previous institution selected when the form submit fails
- Show the red border around the select2 component on error

refs #1352
Simplify registration tests to minimize conflicts and add a few more

Tried to reduce the number of changes to minimize conflicts when merging
upstream. Also added a few more tests for RegistrationsController.

refs #1352