WIP: Configure instance & FE

This adds/exposes the config parameters in the instance config:

• description (new)
  • as site.description in statusnet config (/api/statusnet/config)
  • as description in mastodon api (/api/v1/instance)
  • as metadata.nodeDescription in nodeinfo
• chat_enabled (new)
  • as site.pleromafe.chatDisabled in statusnet config (see below)
• logo (new)
  • as site.pleromafe.logo in statusnet config (see below)
• public
  • as the site.private flag in statusnet config
  • as the metadata.private flag in nodeinfo

This also allows to replace priv/static/static/config.json by site.pleromafe in statusnet config:

config :pleroma, :fe,
  theme: "pleroma-dark",
  background: "/static/aurora_borealis.jpg",
  redirect_root_no_login: "/main/all",
  redirect_root_login: "/main/friends",
  show_instance_panel: true

The format of the JSON object site.pleromafe is the same as the old config.json.

I separated config :pleroma, :instance and config :pleroma, :fe to keep the concerns clear. chat_enabled and logo moved to :instance as they may be useful for something else.