Add "Join Discord" button

Based of the patreon supporter button.
Add a key "discordLink" to the config.json file and set it to your
Discord invite link.

src/ts/server/config.ts

@@ -5,6 +5,7 @@ export interface AppConfig {
 	title: string;
 	twitterLink?: string;
 	supporterLink?: string;
+	discordLink?: string;
 	contactEmail?: string;
 	port: number;
 	adminPort?: number;

src/ts/server/routes/index.ts

@@ -100,6 +100,7 @@ export function createIndex(assetsPath: string, adminAssetsPath: string) {
 			title: config.title,
 			twitterLink: config.twitterLink,
 			supporterLink: config.supporterLink,
+			discordLink: config.discordLink,
 			email: config.contactEmail,
 			logo: `${config.host}${getRevImageURL('logo-120.png')}`,
 			loadingImage: `data:image/png;base64,${loadingImage.toString('base64')}`,