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.

modules/default/updatenotification/git_helper.js

@@ -131,7 +131,7 @@ class GitHelper {
 
 		// this is the default if there was no match from "git fetch -n --dry-run".
 		// Its a fallback because if there was a real "git fetch", the above "git fetch -n --dry-run" would deliver nothing.
-		let refDiff = gitInfo.current + "..origin/" + gitInfo.current;
+		let refDiff = `${gitInfo.current}..origin/${gitInfo.current}`;
 		if (matches && matches[0]) {
 			refDiff = matches[0];
 		}