msgconv: port most of attachment and text message bridging

* 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.

pkg/msgconv/msgconv.go

@@ -17,9 +17,19 @@
 package msgconv
 
 import (
-	"go.mau.fi/mautrix-discord/pkg/connector"
+	"context"
+
+	"go.mau.fi/mautrix-discord/pkg/attachment"
+	"maunium.net/go/mautrix/bridgev2"
 )
 
 type MessageConverter struct {
-	connector *connector.DiscordConnector
+	Bridge *bridgev2.Bridge
+
+	// ReuploadMedia is called when the message converter wants to upload some
+	// media it is attempting to bridge.
+	//
+	// This can be directly forwarded to the ReuploadMedia method on DiscordConnector.
+	// The indirection is only necessary to prevent an import cycle.
+	ReuploadMedia func(ctx context.Context, intent bridgev2.MatrixAPI, portal *bridgev2.Portal, reupload attachment.AttachmentReupload) (*attachment.ReuploadedAttachment, error)
 }