Fix mirrored projects allowing empty import urls
requested to merge 939-repositoryupdatemirrorworker-updatemirrorerror-rugged-invaliderror-cannot-set-empty-url into master
Because of missing validations in the projects model, you could create a new project with a nil or empty import_url with mirror=true. This would cause exceptions when RepositoryUpdateMirrorWorker::UpdateMirrorError ran and attempted to update one of these projects. This was made easier by the fact that you could show the "Mirror repository" checkbox in the UI, check it, hide it, and still have its value considered.
The missing validations have been added, and minor adjustments have been made to the UI to improve the previous behaviour. A migration was also added to update mirror=false in all projects with a nil or empty import url that exists because of the described bug.
Closes #939 (closed)