frontend, python: allow ignoring errors that a project already exists

[FrostyX]

Fix #2140

[FrostyX]

This was much more complicated than I expected.

• @nikromen Am I doing the query parameters parsing correctly? It was done with the RequestParser but that changed in PR frontend: migrate API projects namespace to flask-restx #2807
• I am passing the exist_ok value all the way into the CoprUniqueNameValidator and then hackily trying to access its copr instance from the outside. It works but we may want to do it differently? Maybe not using the validator and instead of CoprsLogic.add using CoprsLogic.get_or_create (we have a similar method for CoprDirsLogic)?

[praiskup]

otherwise lgtm

[FrostyX]

Updated, and I also added a unit test.

[praiskup]

LGTM, thank you!