Make User-Agent configurable (#3255)

Fixes #3253 

Adds a configuration option to overwrite the default `User-Agent` header
that is send at least by the calendar and news module. Allows other
modules to use the individual user agent as well.

The configuration accepts either a string or a function:
```
var config =
	{
		...
		userAgent: 'Mozilla/5.0 (My User Agent)',
		...
	}
```
or
```
var config =
	{
		...
		userAgent: () => 'Mozilla/5.0 (My User Agent)',
		...
	}
```

---------

Co-authored-by: Veeck <github@veeck.de>
Co-authored-by: veeck <gitkraken@veeck.de>
Co-authored-by: Karsten Hassel <hassel@gmx.de>
Co-authored-by: Kristjan ESPERANTO <35647502+KristjanESPERANTO@users.noreply.github.com>

package.json

@@ -26,6 +26,9 @@
 	"imports": {
 		"#module_functions": {
 			"default": "./js/module_functions.js"
+		},
+		"#server_functions": {
+			"default": "./js/server_functions.js"
 		}
 	},
 	"main": "js/electron.js",