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.
* Created a separate discordid package to avoid import cycles.
* Implemented attachment bridging. We still need to implement direct
media, but this will do for now.
* Corrected how encrypted files (e.g. embed images and attachments) were
bridged. Previously, the URL field would be empty.
Still a lot of missing pieces. Thoughts:
* Mentions to roles and custom emoji are not rendered properly. We need
to maintain our own DB.
* We might not need the "attachments" leaf package anymore? It's just
there to avoid an import cycle.
Bridging actual events (i.e. wiring up discordgo's event handlers) is
probably next.
