Configuration option to set all posts as NSFW by default

[kersploosh]

Provide an option to have all posts in a community, or all posts on an entire instance, flagged as NSFW by default.

[Pdzly]

I would recommend to do that for specific users too ( both federated and local ones )

[kgilles]

Note for whoever ends up working on this: When such a community is selected in the post-creation form, we should automatically check the "Is NSFW" checkbox and disable it. Together with a notice explaining why it's happening. To let users know their post will be marked as NSFW by default.