connector: first pass at bridging guilds

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.

pkg/connector/connector.go

@@ -26,6 +26,7 @@ import (
 
 type DiscordConnector struct {
 	Bridge  *bridgev2.Bridge
+	Config  *Config
 	MsgConv *msgconv.MessageConverter
 }