AdminFE functionality in PleromaFE

• Low-hanging fruit instance settings;
  • Instance meta
  • Basic registrations - related stuff (except for welcome messages)
  • Basic "arbitrary limits" stuff (except for federation)
  • VERY Basic frontend management stuff
    • Let you select default FE
    • Lets you install and update FE
    • Adds develop ref for PleromaFE (pleroma#3072)
• Basic AttachmentSetting implementation
• Option and structure to override backend-provided desriptions with frontend ones
• ChoiceSetting support for admin setting
• Adapt modal for admin settings
  • Remove backup dropdown
  • Change title
  • Add AdminFE link
  • Add "this is experimental and WIP" plug
• Add Providers/Consumer to avoid copy-pasting stuff like draft-mode and source="admin" everywhere
• Instead of using local draft state, use shared draft state
  • Add buttons for dumping/resetting entire draft state
  • Use shared draft state for dependencies instead of live state
  • Add group setting for committing/resetting "grouped" states (i.e. restrict_unauthenticated -> :timelines)
• Handle case where database config isn't enabled gracefully.

Current plan for tabs:

• Instance
  • Meta
  • Registrations
  • Access
• Invites
• Limits
  • Arbitrary limits (i.e. post count)
  • Rate limits
• Federation
• Processing
  • General status processing (i.e. attachment links)
  • Media Proxy
  • Link Formatter
  • OpenGraph/TwitterCard
  • MRF
• Mailer
• Integrations
  • Prometheus
  • LDAP
  • BBS/SSH
  • Upload
• Tuning
  • Job queues