Create a debug screen with a Webview showing possible links handled by the app

_Originally posted by @ouchadam in https://github.com/vector-im/element-android/issues/4335#issuecomment-951159285_

> for the deeplink testing, something that worked well for me in the past is to include a html file with all the combinations of links in the debug app and load it into a webview

Maybe this new Activity can be set with a launcher so we can have a dedicated task running only this Activity.