Add insert sticker shortcut

[aude]

No description provided.

[bhoomikashree]

Add private mode

[sindresorhus]

Now I'm planning to do this after #1013

It's merged.

[aude]

Thanks.

PR updated.

[CvX]

On the old UI the first triggering of the shortcut results with just a flash of the sticker selector. Further shortcut invocations seem to work:

Unfortunately the #941 workaround for emoji doesn't work for this issue.
TBH I don't think it's a problem, I'd merge it in anyway.

[aude]

Thanks for checking.

I remember that being a problem, it resembles the emoji problem to me.

As a side note, the trigger is a tiny bit weird trigger in the new UI as well. It might be because the code is clicking the <a>, but if you do it with the mouse you're actually clicking the <svg> (or elements inside). I tried simulating clicking the SVG using this method, and it actually worked better.

If you want, @CvX, you could check whether that method works better in the old UI as well. But I'm open to merging this PR like it is.