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/discordid/id.go

@@ -0,0 +1,35 @@
+// mautrix-discord - A Matrix-Discord puppeting bridge.
+// Copyright (C) 2024 Tulir Asokan
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package discordid
+
+import (
+	"github.com/bwmarrin/discordgo"
+	"maunium.net/go/mautrix/bridgev2/networkid"
+)
+
+func MakePortalKey(ch *discordgo.Channel, userLoginID networkid.UserLoginID, wantReceiver bool) (key networkid.PortalKey) {
+	key.ID = networkid.PortalID(ch.ID)
+	if wantReceiver {
+		key.Receiver = userLoginID
+	}
+	return
+}
+
+func MakePortalKeyWithID(channelID string) (key networkid.PortalKey) {
+	key.ID = networkid.PortalID(channelID)
+	return
+}