Add support for custom emoji in reactions.

This seems to be working correctly, but element-desktop isn't rendering them,
not sure if that's expected or not.

Closes #4

database/emojiquery.go

@@ -0,0 +1,44 @@
+package database
+
+import (
+	log "maunium.net/go/maulogger/v2"
+
+	"maunium.net/go/mautrix/id"
+)
+
+type EmojiQuery struct {
+	db  *Database
+	log log.Logger
+}
+
+const (
+	emojiSelect = "SELECT discord_id, discord_name, matrix_url FROM emoji"
+)
+
+func (eq *EmojiQuery) New() *Emoji {
+	return &Emoji{
+		db:  eq.db,
+		log: eq.log,
+	}
+}
+
+func (eq *EmojiQuery) GetByDiscordID(discordID string) *Emoji {
+	query := emojiSelect + " WHERE discord_id=$1"
+
+	return eq.get(query, discordID)
+}
+
+func (eq *EmojiQuery) GetByMatrixURL(matrixURL id.ContentURI) *Emoji {
+	query := emojiSelect + " WHERE matrix_url=$1"
+
+	return eq.get(query, matrixURL.String())
+}
+
+func (eq *EmojiQuery) get(query string, args ...interface{}) *Emoji {
+	row := eq.db.QueryRow(query, args...)
+	if row == nil {
+		return nil
+	}
+
+	return eq.New().Scan(row)
+}