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/modules/clock_spec.js

@@ -89,7 +89,7 @@ describe("Clock module", () => {
 
 		it("should show the week with the correct number of week of year", async () => {
 			const currentWeekNumber = moment().week();
-			const weekToShow = "Week " + currentWeekNumber;
+			const weekToShow = `Week ${currentWeekNumber}`;
 			const elem = await helpers.waitForElement(".clock .week");
 			expect(elem).not.toBe(null);
 			expect(elem.textContent).toBe(weekToShow);