Small fix in SpacesHelper#space_join_button after 5ae7b41c0

refs #1067
Better redirects when a user has pending join requests

When a user tries to access a space and gets a 403:
* If he already asked to join the space, show the new join request page with
  a warning that he already sent a join request
* If the was already invited to join the space, redirect to the page to accept
  or decline the join request
* No join request pending, send him to create a new join request
* No user logged in, show the 403 error

refs #1067
Disable new join request form if the user already has an invitation

Was already being disabled if the user had a request, but not for an
invitation.

refs #1067
Notify space admins in the sidebar when there are pending join requests

refs #1067