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/fonts_spec.js

@@ -3,7 +3,7 @@ const helpers = require("./helpers/global-setup");
 describe("All font files from roboto.css should be downloadable", () => {
 	const fontFiles = [];
 	// Statements below filters out all 'url' lines in the CSS file
-	const fileContent = require("fs").readFileSync(__dirname + "/../../fonts/roboto.css", "utf8");
+	const fileContent = require("fs").readFileSync(`${__dirname}/../../fonts/roboto.css`, "utf8");
 	const regex = /\burl\(['"]([^'"]+)['"]\)/g;
 	let match = regex.exec(fileContent);
 	while (match !== null) {
@@ -21,7 +21,7 @@ describe("All font files from roboto.css should be downloadable", () => {
 	});
 
 	test.each(fontFiles)("should return 200 HTTP code for file '%s'", async (fontFile) => {
-		const fontUrl = "http://localhost:8080/fonts/" + fontFile;
+		const fontUrl = `http://localhost:8080/fonts/${fontFile}`;
 		const res = await helpers.fetch(fontUrl);
 		expect(res.status).toBe(200);
 	});