Add user setting to opt-out from data collection

[ChristophWurst]

For https://github.com/nextcloud/mail/projects/7.

User should have control over whether their interactions are recorded by the Nextcloud instance or not. We will never send it anywhere else, but still the user has to have the choice.

Thus we need a new checkbox in the app's settings menu, just like we have for the gravatars, and persist the opt-out on the server.

@GretaD can you add the new checkbox and wire it up with the back-end just like we do with the gravatar setting? I can take care of the backe-end changes, if any necessary. :)

[ChristophWurst]

And we should then already use this setting for the existing recipient collection code.