Improve CODEOWNERS code mapping creation flow

Steps: - The customer goes to ownership rules - since they have source code integrations they can click on "Import CODEOWNERS" [1] - they pick their integration [2] - they're redirected to the code mappings tab [3] - they manually enter the values and hit save Issues noticed: - No validation is made if the values are correct and there's a CODEOWNERS - If the values are incorrect the ownership rules will be empty [4] - In my case, I picked `master` instead of `develop` - The customer should have picked (`foo/`, ` `) but they chose (` `, `bar/`) which caused - Auto assignment did not work - Stack trace links did not work [1]  [2]  [3]  [4]  [5]