New weather provider: Yr.no (#2948)

# Added Yr.no as a weather provider

Yr.no is a free Norwegian weather service. The configuration is quite
simple:
```js
{
    weatherProvider: "yr",
    lat: 59.9171,
    lon: 10.7276,
    altitude: 30
}
```
The latitude and longitude cannot have more than 4 decimals, but that
should be plenty. To quote yr: "There is no need to ask for weather
forecasts with nanometer precision!". The altitude should be meters
above sea level and defaults to 0. If `type` is set to `current` the
symbol can display the next 1, 6 or 12 hours by setting
`currentForecastHours` (default is 1).

It states in [Getting
started-guide](https://developer.yr.no/doc/GettingStarted/) that users
of the API should cache the results and use the `Expires`-header to know
when to ask for new data. By using the `If-Modified-Since`-header we can
avoid downloading the same data over and over again. I chose not to
override the `User-Agent`-header set in
[`server.js`](https://github.com/MichMich/MagicMirror/blob/a328ce5/js/server.js#L97)
even though it does not comply with [the terms of
service](https://developer.yr.no/doc/TermsOfService/). It currently
works with the default header, and by searching the web for MagicMirror
the GitHub-repo should be easy to find without an explicit link.

I also had to make some minor changes to `server.js` and
`weatherprovider.js` to be able to send and return HTTP headers. To
handle the HTTP 304 response without body I chose to return `undefined`
so we easily can use the response as a condition: `if (response) ...`.

The documentation for the API is available here:
- [API Reference overview](https://api.met.no/weatherapi/)
-
[Locationforecast](https://api.met.no/weatherapi/locationforecast/2.0/)
- Used to get the weather forecast
- [Sunrise](https://api.met.no/weatherapi/sunrise/2.0/documentation) -
used to find sunrise and sunset times

Co-authored-by: Veeck <github@veeck.de>

modules/default/weather/weatherprovider.js

@@ -1,4 +1,4 @@
-/* global Class */
+/* global Class, performWebRequest */
 
 /* MagicMirror²
  * Module: Weather
@@ -111,36 +111,23 @@ const WeatherProvider = Class.extend({
 		this.delegate.updateAvailable(this);
 	},
 
-	getCorsUrl: function () {
-		if (this.config.mockData || typeof this.config.useCorsProxy === "undefined" || !this.config.useCorsProxy) {
-			return "";
-		} else {
-			return location.protocol + "//" + location.host + "/cors?url=";
-		}
-	},
-
 	/**
 	 * A convenience function to make requests.
 	 *
 	 * @param {string} url the url to fetch from
 	 * @param {string} type what contenttype to expect in the response, can be "json" or "xml"
+	 * @param {Array.<{name: string, value:string}>} requestHeaders the HTTP headers to send
+	 * @param {Array.<string>} expectedResponseHeaders the expected HTTP headers to recieve
 	 * @returns {Promise} resolved when the fetch is done
 	 */
-	fetchData: async function (url, type = "json") {
-		url = this.getCorsUrl() + url;
+	fetchData: async function (url, type = "json", requestHeaders = undefined, expectedResponseHeaders = undefined) {
 		const mockData = this.config.mockData;
 		if (mockData) {
 			const data = mockData.substring(1, mockData.length - 1);
 			return JSON.parse(data);
-		} else {
-			const response = await fetch(url);
-			const data = await response.text();
-			if (type === "xml") {
-				return new DOMParser().parseFromString(data, "text/html");
-			} else {
-				return JSON.parse(data);
-			}
 		}
+		const useCorsProxy = typeof this.config.useCorsProxy !== "undefined" && this.config.useCorsProxy;
+		return performWebRequest(url, type, useCorsProxy, requestHeaders, expectedResponseHeaders);
 	}
 });