added default payload to emails, and adding nodebb logo for email header

src/emailer.js

@@ -25,10 +25,17 @@ var fallbackTransport;
 
 var Emailer = module.exports;
 
+Emailer._defaultPayload = {};
 
 Emailer.registerApp = function (expressApp) {
 	app = expressApp;
 
+	Emailer._defaultPayload = {
+		url: nconf.get('url'),
+		site_title: meta.config.title || 'NodeBB',
+		'brand:logo': nconf.get('url') + meta.config['brand:logo'],
+	};
+
 	// Enable Gmail transport if enabled in ACP
 	if (parseInt(meta.config['email:GmailTransport:enabled'], 10) === 1) {
 		transports.gmail = nodemailer.createTransport(smtpTransport({
@@ -55,6 +62,9 @@ Emailer.send = function (template, uid, params, callback) {
 		return callback();
 	}
 
+	// Combined passed-in payload with default values
+	params = Object.assign({}, Emailer._defaultPayload, params);
+
 	async.waterfall([
 		function (next) {
 			async.parallel({