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.

tests/e2e/helpers/global-setup.js

@@ -28,7 +28,7 @@ exports.stopApplication = async () => {
 
 exports.getDocument = () => {
 	return new Promise((resolve) => {
-		const url = "http://" + (config.address || "localhost") + ":" + (config.port || "8080");
+		const url = `http://${config.address || "localhost"}:${config.port || "8080"}`;
 		jsdom.JSDOM.fromURL(url, { resources: "usable", runScripts: "dangerously" }).then((dom) => {
 			dom.window.name = "jsdom";
 			dom.window.fetch = corefetch;