Use template literals instead of string concatenation (#3066)

We have used it inconsistently till now. Template literals are more
modern and easier to maintain in my opinion.

Because that's a large amount of changes, here's a way to reproduce it:
I added the rule `"prefer-template": "error"` to the `.eslintrc.json`
and did an autofix. Since this caused a new problem in line 409 of
`newsfeed.js`, I reversed it in that line and also removed the rule from
the eslint config file.

The rule is described here:
https://eslint.org/docs/latest/rules/prefer-template

Note: I've played around with some other linter rules as well, and some
seem to point to some specific, non-cosmetic, issues. But before I dive
even deeper and then introduce even bigger and hardly understandable
changes at once, I thought I'd start with this simple cosmetic rule.

js/socketclient.js

@@ -18,8 +18,8 @@ const MMSocket = function (moduleName) {
 	if (typeof config !== "undefined" && typeof config.basePath !== "undefined") {
 		base = config.basePath;
 	}
-	this.socket = io("/" + this.moduleName, {
-		path: base + "socket.io"
+	this.socket = io(`/${this.moduleName}`, {
+		path: `${base}socket.io`
 	});
 
 	let notificationCallback = function () {};