Fix bookmarklet on mobile browsers, eg Firefox

[snarfed]

First off, thanks for maintaining Press This, it's great!

The bookmarklets stopped working for me in Firefox Mobile a while back; background in https://github.com/mozilla-mobile/fenix/issues/2871#issuecomment-1160501912. Turns out opening a new window was the culprit. I fixed it for myself by removing lines 149 (target) and 152 (window.open) here so that it opens in the current window instead:

https://github.com/WordPress/press-this/blob/9ca88faabbf9e7eb6595a6b5da3b9120becf2ab6/assets/bookmarklet.js#L149-L152

We could just maintain a separate copy for mobile browser, but obviously it'd be better to keep them unified. I don't know how to sniff for mobile browsers though. Do you? Any idea how you'd want to incorporate this?

Thanks in advance!