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/electron/modules/calendar_spec.js

@@ -7,7 +7,7 @@ describe("Calendar module", () => {
 	 * @param {string} cssClass css selector
 	 */
 	const doTest = async (cssClass) => {
-		let elem = await helpers.getElement(".calendar .module-content .event" + cssClass);
+		let elem = await helpers.getElement(`.calendar .module-content .event${cssClass}`);
 		expect(await elem.isVisible()).toBe(true);
 	};