send the banned user an email with reason + time limit if applies

public/language/en-GB/email.json

@@ -44,5 +44,10 @@
 
 	"unsub.cta": "Click here to alter those settings",
 
+	"banned.subject": "You have been banned from %1",
+	"banned.text1": "The user %1 has been banned from %2.",
+	"banned.text2": "This ban will last until %1.",
+	"banned.text3": "This is the reason why you have been banned:",
+
 	"closing": "Thanks!"
 }

src/socket.io/user/ban.js

@@ -3,10 +3,12 @@
 var async = require('async');
 
 var user = require('../../user');
+var meta = require('../../meta');
 var websockets = require('../index');
 var events = require('../../events');
 var privileges = require('../../privileges');
 var plugins = require('../../plugins');
+var emailer = require('../../emailer');
 var translator = require('../../translator');
 
 module.exports = function (SocketUser) {
@@ -97,6 +99,22 @@ module.exports = function (SocketUser) {
 				if (isAdmin) {
 					return next(new Error('[[error:cant-ban-other-admins]]'));
 				}
+
+				user.getUserField(uid, 'username', next);
+			},
+			function (username, next) {
+				var siteTitle = meta.config.title || 'NodeBB';
+				var data = {
+					subject: '[[email:banned.subject, ' + siteTitle + ']]',
+					site_title: siteTitle,
+					username: username,
+					until: until ? new Date(until).toString() : false,
+					reason: reason,
+				};
+
+				emailer.send('banned', uid, data, next);
+			},
+			function (next) {
 				user.ban(uid, until, reason, next);
 			},
 			function (next) {

src/views/emails/banned.tpl

@@ -0,0 +1,18 @@
+<p>
+	[[email:banned.text1, {username}, {site_title}]]
+	<!-- IF until -->
+	[[email:banned.text2, {until}]]
+	<!-- ENDIF until -->
+</p>
+
+<!-- IF reason -->
+<p>
+	[[email:banned.text3]]
+</p>
+
+<p>
+	{reason}
+</p>
+<!-- ENDIF reason -->
+
+<!-- IMPORT emails/partials/footer.tpl -->