In the same vein as mautrix-whatsapp, -slack and others, do not make
assumptions about how the ID is represented in the connector code. Let
the discordid package be entirely responsible.
* 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.
