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

bridge/emoji.go

@@ -0,0 +1,53 @@
+package bridge
+
+import (
+	"io/ioutil"
+	"net/http"
+
+	"github.com/bwmarrin/discordgo"
+
+	"maunium.net/go/mautrix/appservice"
+	"maunium.net/go/mautrix/id"
+)
+
+func (p *Portal) downloadDiscordEmoji(id string, animated bool) ([]byte, string, error) {
+	var url string
+	var mimeType string
+
+	if animated {
+		// This url requests a gif, so that's what we set the mimetype to.
+		url = discordgo.EndpointEmojiAnimated(id)
+		mimeType = "image/gif"
+	} else {
+		// This url requests a png, so that's what we set the mimetype to.
+		url = discordgo.EndpointEmoji(id)
+		mimeType = "image/png"
+	}
+
+	req, err := http.NewRequest(http.MethodGet, url, nil)
+	if err != nil {
+		return nil, mimeType, err
+	}
+
+	req.Header.Set("User-Agent", discordgo.DroidBrowserUserAgent)
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return nil, mimeType, err
+	}
+
+	defer resp.Body.Close()
+
+	data, err := ioutil.ReadAll(resp.Body)
+
+	return data, mimeType, err
+}
+
+func (p *Portal) uploadMatrixEmoji(intent *appservice.IntentAPI, data []byte, mimeType string) (id.ContentURI, error) {
+	uploaded, err := intent.UploadBytes(data, mimeType)
+	if err != nil {
+		return id.ContentURI{}, err
+	}
+
+	return uploaded.ContentURI, nil
+}