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/emoji.go

@@ -0,0 +1,70 @@
+package database
+
+import (
+	"database/sql"
+	"errors"
+
+	log "maunium.net/go/maulogger/v2"
+
+	"maunium.net/go/mautrix/id"
+)
+
+type Emoji struct {
+	db  *Database
+	log log.Logger
+
+	DiscordID   string
+	DiscordName string
+
+	MatrixURL id.ContentURI
+}
+
+func (e *Emoji) Scan(row Scannable) *Emoji {
+	var matrixURL sql.NullString
+	err := row.Scan(&e.DiscordID, &e.DiscordName, &matrixURL)
+
+	if err != nil {
+		if !errors.Is(err, sql.ErrNoRows) {
+			e.log.Errorln("Database scan failed:", err)
+		}
+
+		return nil
+	}
+
+	e.MatrixURL, _ = id.ParseContentURI(matrixURL.String)
+
+	return e
+}
+
+func (e *Emoji) Insert() {
+	query := "INSERT INTO emoji" +
+		" (discord_id, discord_name, matrix_url)" +
+		" VALUES ($1, $2, $3);"
+
+	_, err := e.db.Exec(query, e.DiscordID, e.DiscordName, e.MatrixURL.String())
+
+	if err != nil {
+		e.log.Warnfln("Failed to insert emoji %s: %v", e.DiscordID, err)
+	}
+}
+
+func (e *Emoji) Delete() {
+	query := "DELETE FROM emoji WHERE discord_id=$1"
+
+	_, err := e.db.Exec(query, e.DiscordID)
+	if err != nil {
+		e.log.Warnfln("Failed to delete emoji %s: %v", e.DiscordID, err)
+	}
+}
+
+func (e *Emoji) APIName() string {
+	if e.DiscordID != "" && e.DiscordName != "" {
+		return e.DiscordName + ":" + e.DiscordID
+	}
+
+	if e.DiscordName != "" {
+		return e.DiscordName
+	}
+
+	return e.DiscordID
+}