Purge groups and roles from DB (for real)

[davelopez]

Fixes #15350

In addition to removing the purged groups and roles from DB. Increases a bit the test coverage, includes some small refactorings, and makes some error codes more explicit and consistent between both APIs:

• Returning 409 (Conflict) when creating/updating a role and the name is already in use.
• Returning 404 (instead of 400) when the role doesn't exist or the user doesn't have access to it.

How to test the changes?

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[jmchilton]

Very nice