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]