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/configs/empty_ipWhiteList.js

@@ -3,7 +3,7 @@
  * By Rodrigo Ramírez Norambuena https://rodrigoramirez.com
  * MIT Licensed.
  */
-let config = require(process.cwd() + "/tests/configs/default.js").configFactory({
+let config = require(`${process.cwd()}/tests/configs/default.js`).configFactory({
 	ipWhitelist: []
 });