Improve error reporting on users API

* users (#6878, #3526, #4209): Validation error messages are now exposed through 400 responses, 409 response are sent in case of duplicate email or username
* MRs (#5335): 409 responses are sent in case of duplicate merge request (source/target branches), 422 responses are sent when submiting MR fo/from unrelated forks
* issues
* labels
* projects
Merge branch 'api/improve-error-reporting' of https://github.com/jubianchi/gitlabhq into jubianchi-api/improve-error-reporting
Add notice about weekend in release cycle.
Add GitAccess specs
Refactor Gitlab::GitAccess tests
Merge branch 'git-access-tests' into 'master'

Gitlab::GitAccess tests

Fixes #1616

See merge request !1113
Merge branch 'jubianchi-api/improve-error-reporting' into 'master'

Merge jubianchi-api/improve-error-reporting

Closes https://github.com/gitlabhq/gitlabhq/pull/7502

See merge request !1112
Merge branch 'master' of github.com:gitlabhq/gitlabhq