msgconv: port the majority of embed and attachment bridging

We still need to implement direct media support, so for now we encrypt
and reupload to Matrix. Notably remaining is conversion to HTML.

pkg/msgconv/msgconv.go

@@ -16,8 +16,10 @@
 
 package msgconv
 
-import "maunium.net/go/mautrix/bridgev2"
+import (
+	"go.mau.fi/mautrix-discord/pkg/connector"
+)
 
 type MessageConverter struct {
-	bridge *bridgev2.Bridge
+	connector *connector.DiscordConnector
 }