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/weather/providers/ukmetoffice.js

@@ -195,8 +195,8 @@ WeatherProvider.register("ukmetoffice", {
 	 */
 	getParams(forecastType) {
 		let params = "?";
-		params += "res=" + forecastType;
-		params += "&key=" + this.config.apiKey;
+		params += `res=${forecastType}`;
+		params += `&key=${this.config.apiKey}`;
 		return params;
 	}
 });