By not implementing MetaMerger, UserLogin metadata such as
BridgedGuildIDs would get clobbered upon reauthing because the metadata
was replaced entirely. Implement CopyFrom so we can gain deeper control
over what is preserved upon reauth. Notably, preserve BridgedGuildIDs so
we can't get into a weird state where a guild is bridged but
simultaneously absent from BridgedGuildIDs, which would cause us to not
subscribe (OP 14) to it properly.
For each guild specified in the config, create a space and bridge all
contained text channels that the user has permissions to view.
* Finally add a custom config struct where we accept a list of guild IDs
to bridge. This is intended to be temporary as we flesh out the proper
interfaces for managing which guilds to bridge.
* Defined a custom meta type for portals that holds the containing guild
ID of the channel (if any).
* Transferred the responsibility of building a channel's ChatInfo and
ChatMemberList to the DiscordChatResync event itself.
