Replace API and OAuth URLs in settings by a simple Sandbox checkbox

Will wait for this issue (CaptchetatService) to be merged before pushing this issue's MR.

I have thoroughly tested the changes, and it works as expected. The following tests were conducted:

• Basic functionality tests
• Compatibility tests with other modules

No issues were found, and the changes are ready to be merged.

@ewehbe should be ok to merge: tested successfully with sandbox and production credentials.