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/basic-auth.js

@@ -12,7 +12,7 @@ app.use(basicAuth);
 
 // Set available directories
 const directories = ["/tests/configs", "/tests/mocks"];
-const rootPath = path.resolve(__dirname + "/../../../");
+const rootPath = path.resolve(`${__dirname}/../../../`);
 
 for (let directory of directories) {
 	app.use(directory, express.static(path.resolve(rootPath + directory)));