changes the usage of simpleprojectdetails to already implemented basicprojectsdetails and changes the url to a more reader friendly format

1. Return 404 if commit is not found (RESTful resource not found)
2. Return an empty array if pipeline is not found (resource present, no
   associated builds found)
3. Return an empty array if pipeline found but no builds there (resource
   present, no associated builds)

rely only on IssuesFinder
docs and changelog

This commit builds on the groundwork in
ee008e300b1ec0abcc90e6a30816ec0754cea0dd, which refactored the backend
so the same code could be used for new dropdowns. In this commit its
used for templates for the `.gitlab-ci.yml` files.

Docs also added.

This only supports Issues and MergeRequests right now because of the
consistency of the routes those models provide.

This reverts commit 13e37a3e.

- Extract a duplicated `redirect_to`
- Fix a typo: "token", not "certificate"
- Have the "Expires at" datepicker be attached to a text field, not inline
- Have both private tokens and personal access tokens verified in a
  single "authenticate_from_private_token" method, both in the
  application and API. Move relevant logic to
  `User#find_by_personal_access_token`
- Remove unnecessary constants relating to API auth. We don't need a
  separate constant for personal access tokens since the param is the
  same as for private tokens.

This reverts commit 06e0ef07

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4641#note_12444891