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/discordid/dbmeta.go

@@ -18,6 +18,15 @@ package discordid
 
 import "github.com/bwmarrin/discordgo"
 
+type PortalMetadata struct {
+	// The ID of the Discord guild that the channel corresponding to this portal
+	// belongs to.
+	//
+	// For private channels (DMs and group DMs), this will be the zero value
+	// (an empty string).
+	GuildID string `json:"guild_id"`
+}
+
 type UserLoginMetadata struct {
 	Token            string                     `json:"token"`
 	HeartbeatSession discordgo.HeartbeatSession `json:"heartbeat_session"`