Make it possible to lock runner on a specific project
Make it possible to lock runner on a specific project.

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Closes#3407
See merge request !4093
* master:
Move CI admin builds and runners specs to correct directory
Fix 500 when viewing specific runners on runners page
Fix Ci::Project migration not migrating columns that cannot be NULL
Fix MySQL migration of CI emails
Minor fix in flow 'Merge when build succeeds'