Replace deprecated String.prototype.substr()

String.prototype.substr() is deprecated (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr) so we replace it with slice() or substring() which work similarily but aren't deprecated.
Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

src/main/base/browserwindow.ts

@@ -695,7 +695,7 @@ export class BrowserWindow {
 
             let latestbranchjson = await latestbranch.json()
             let base_url = latestbranchjson[0].url
-            base_url = base_url.substr(0, base_url.lastIndexOf('/'))
+            base_url = base_url.substring(0, base_url.lastIndexOf('/'))
 
             const options: any = {
                 provider: 'generic',