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/example-config.yaml

@@ -0,0 +1,6 @@
+# Configuration options related to Discord guilds (also known as "servers").
+guilds:
+    # UNSTABLE: The IDs of the guilds to bridge. This is a stopgap measure
+    # during bridge development. If no guild IDs are specified, then no guilds
+    # are bridged at all.
+    bridging_guild_ids: []