Minimal scopes mode

Original proposition, mostly done.

Currently, option scopeOptionsEnabled exists mostly because of compatibility with GS reasons, however some people still want in on "off" even with pleroma backend to prevent users from using some of the options that considered harmful or otherwise advertised against using them by the instance admin.

• The proposition is to move from scopeOptionsEnabled (and assume it's always true) to scopeOptions which reports an array of supported scopes: ['direct','followers-only','unlisted','public'] removing some of them will make FE to stop showing specific option. FE might also stop using these modes and instead "upgrade" them to 'public' (except for 'direct'). BE might implement some sort of mechanism to prevent creation of said posts.

• Another option that should be present is scopeCopy - user-configurable with instance-configurable default. Toggle that copies scope when replying to a post, i.e. should reply to a 'followers-only' post also be 'followers-only' or default ('public' or allow to configure default too). SHOULD NOT AFFECT DMs

• Subject line should be (user, instance defaults) configurable too:

  • Toggle display of subject line
  • Copy behavior: email style (current one, with re:'s), mastodon style (copy as is), don't copy When subject line hidden by default it still can be shown when copying replies, it should also have a "clear" button to manually clear (and hide)

Since the last time I've edited this issue, some things changed.

• When replying to a non-public post with a post form set to a less stricter scope (i.e. replying to a FO post with Public post either because user explicitly set scope to public or because scopeCopy is set to false) FE should display a warning that you're increasing scope level. It should be possible to disable warnings separately for FO and for Unlisted posts.
• In addition to above, another (also disabable) warning should appear when scopeCopy is false. Something that should notify people of this option existence if it's disable by default instance-wise.
• "Minimal scopes" mode option: Instead of showing all scope options in post form only show: DM, user's default scope, scope of post replying to if it's different.
• For DMs only show DM scope, ever.