Improve error messages when squashing fails

Closes gitlab-com/support-forum#1568

See merge request !1318

Match require statement. Status code is now undefined in master

See merge request !1314

Add MockCiService integration

See merge request !1310

The `find_merge_source` method was added for CE compatibility, but 'no
source for merge' is not a useful message for users. Use the actual
message returned from the squash service in this case, and a custom
exception to save that on the model.

* master:
  Fixed failing tests with ajax spinner
  API: Return 400 for all validation erros in the mebers API
  Remove repository_storage from V4 "/application/settings" settings API
  Add repository storage field back to projects API

Remove repository_storage from V4 "/application/settings" settings API

Closes #1790

See merge request !1307

Add repository storage field back to projects API

See merge request !1300

Fixed failing tests with ajax spinner

See merge request !1317

# Conflicts:
#   doc/api/v3_to_v4.md

API: Return 400 for all validation erros in the mebers API

See merge request !1311

Port of "remove-require-context-application-bundle" to EE

See merge request !1298

Deploy boards

Closes #1589

See merge request !1233

MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9250/

See https://gitlab.com/madlittlemods/gl-mock-ci-service

Conflicts:
	lib/api/services.rb

Port of dm-fix-web-edit-new-lines to EE

See merge request !1248

remove Turbolinks from lockfile.js, do page refresh instead

See merge request !1299

Enable the RSpec/BeEql cop

See merge request !1295

This was an admin-only field that was exposed, and there were no tests
for doing this as an admin and getting that response. In CE, we renamed
the `user` option to `current_user`, and inadvertently broke this.

Deleting a user shouldn't delete associated issues.

See merge request !862

* master:
  Remove ProjectGitHook API
  Document how to use filter-branch to reduce the repo file size

Remove ProjectGitHook API

Closes #1791

See merge request !1301

Document how to use filter-branch to reduce the repo file size

Closes #1270

See merge request !914

[ci skip]