implement custom CSS for subplebbits (text colors, banner, background etc) and settings for it

[plebeius-eth]

add respective subplebbit settings for the owner, and custom UI checkbox in the seedit preferences for the user.

from the API:

SubplebbitSuggested { // values suggested by the sub owner, the client/user can ignore them without breaking interoperability
  primaryColor?: string
  secondaryColor?: string
  avatarUrl?: string
  bannerUrl?: string
  backgroundUrl?: string
  language?: string
  // TODO: menu links, wiki pages, sidebar widgets
}