Use subplebbit.suggested.avatarUrl and subplebbit.suggested.bannerUrl for subplebbit images

[estebanabaroa]

The subplebbit object has some image URL properties that can be configured:

subplebbit.suggested? = {
  primaryColor?: string
  secondaryColor?: string
  avatarUrl?: string
  bannerUrl?: string
  backgroundUrl?: string
}

There are already places in the UI for avatarUrl and bannerUrl so they should be used already. The mock content already has these values and some of the default subs also have them like https://ipfsgateway.xyz/ipns/12D3KooWNMybS8JqELi38ZBX897PrjWbCrGoMKfw3bgoqzC2n1Dh