Allow admins to disable the newsletter tab

[marienfressinaud]

Is your feature request related to a problem? Please describe.

The "newsletter" tab is pretty complicated to end-users and expose some sensitive configuration.

Describe the solution you'd like

It would be nice to allow administrators to disable the newsletter on their server, so they don't have to bother with support requests about this feature.

Describe alternatives you've considered

None

Additional context

I already have a patch working at master...marienfressinaud:Pytition:new/disable-newsletter, I wonder if you would be interested to integrate it upstream?

[fallen]

@marienfressinaud this looks nice, why not! Lemme do some testing to see if it does not break something but yes why not!
Sorry for not answering sooner!