Hide participants from other regforms

[vtran99]

Request to add new privacy setting for Participant List: setting to hide participants from other registration forms. This PR replaces https://github.com/indico/indico/pull/6167, the code is done following previous review from Duarte.

Note: I will fix the privacy_test.py once we agree on the code change, thanks.

[duartegalvao]

Overall the changes look good! You can go ahead and fix the tests (and maybe add some to test this new behavior). When you're done let me know so I can take a closer look / test the PR

[vtran99]

I fixed privacy tests and added some for the new feature, please check.