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
Instead of a `user_tmp` and redirecting to `register_path` here, have you tried to set the error in the user and render the `new` view? Something like:

```ruby
if params[:user][:institution_id].blank? or !Institution.where(id: params[:user][:institution_id]).first
  @user.errors.add(:institution_id, "can't be blank")
  render :new
end
```

Like we do in [other controllers](https://github.com/mconf/mconf-web/blob/master/app/controllers/spaces_controller.rb#L113).

Also, devise does something a bit different, it calls `respond_with resource` ([see this](https://github.com/plataformatec/devise/blob/master/app/controllers/devise/registrations_controller.rb#L37)). So another idea (probably a better one) is to set the error on `resource` and call that `respond_with resource`.