Display user's pending requests in manage profile

- Implement User#pending_contributor_requests, which returns all of the pending
  contributor requests for a user.
Display requests to join an organization

- Create OrganizationAuthorizer#manage_requests_to_join
- Add missing # in OrganizationAuthorizer.
- Create requests_to_join action for Organizations
- Constrain requests to join route to env enablement
- Implement Organization#pending_requests_to_join
- CclaSignaturesHelper#organization_tab supports rel
- Add Requests to Join to ccla sig/org tab nav
- Constrain Requests to Join tab to env enablement
Merge pull request #630 from opscode/display_pending_requests

Display pending requests