Ability for users to mark posts as sensitive by default

Related to: pleroma#2459

Basically allow users to have the option to mark their posts as NSFW/sensitive by default, though still allowing a manual override if so desired.

Feld suggested that it could be done client-side, so I'm opening an issue here. One potential issue is that this setting might not persist between different devices/clients the user may have open, but it would still be useful anyways.