Merge branch 'fix/import-url-validator' into 'master'

Fixing URL validation for import_url on projects

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/17536

This MR fixes problems related to bypassing `import_url` validation on projects. This makes sure the URL is properly validated so we don't enter crap and fail while running workers that handle this URL.

It also adds a migration to fix current invalid `import_url`s

See merge request !4753