Support mentions

[instantiator]

We should create a CommonMention containing:

• Dictionary<SocialNetwork, account> NetworkToAccount
  • Key = a target network
  • Value = a target account

When generating links, the context network and target network should be considered. ie. If the context network is AT, and the target network is Mastodon, the CommonMention should resolve to a link Facet; whereas if both context and target are AT, the CommonMention should resolve to a 'native' mention Facet.

We'll need an IAccountLinkGenerator generator for each network that can create external links for consumption by the context network as it consumes CommonMentions.