Keep masking back in `Content`, since the scaling animation is happening
on `Content` instead of `this`. This doesn't regress the intended
behaviour in this PR (which is to just to make the button class itself
sheared instead of its content).
As in, `https://osu.ppy.sh/multiplayer/rooms/{id}` links, clicked from
the chat overlay, now directly open in the client.
Additionally, `osu://room/{id}` can be used in the same way to open a
room from a third-party application or a browser.