[Fix] Adds scrolling to description popover

[mattnotmitt]

Changes data-trigger to focus so scrolling works, sets the max height and enables scrollbar.
Fixes #137.

[n1474335]

Thanks for taking a look at this. There are a couple of undesirable behaviours associated with it at the moment which I can't quite iron out.

• It would be nice if popovers were still triggered by hover instead of having to focus them first. I find that most people don't notice or read the descriptions otherwise. Happy to debate this though - are the popups annoying for anyone? Can we think of a nicer way of displaying the descriptions that gets in people's faces, but not in a frustrating way?
• Adding overflow-y: auto to the .popover class makes the arrow tag fall out the bottom instead of being displayed on the side next to the operation it refers to. The fix for this is to add the overflow to the .popover-content class instead, but this doesn't seem to work as expected.

[mattnotmitt]

You can't have hover and scrolling at the same time because as soon as you stop hovering over the element it disappears, and if you try scrolling when you're hovering it just scrolls the list of operations! I'm not sure what the solution to this is though. I'll fix the overflow/arrow issue shortly, however.

[n1474335]

I agree this is not possible without modifications to the way Bootstrap handles the hover event. There are some possible solutions here that might work: https://stackoverflow.com/questions/7703878/how-can-i-hold-twitter-bootstrap-popover-open-until-my-mouse-moves-into-it

[mattnotmitt]

Right, I've fixed the arrow issue. When fixing this, I was thinking that maybe we should set the max-height to 80 or 90%, I think it looks less cramped than having it flow all the way from the top to the bottom of the screen. I've set this at 90% in the latest commit, so maybe I could get a few second opinions.

In regards to the hover event issue; to use the solutions seen on SO we'd have to run it every time the list of operations updates - I'm not particularly sure how I can catch this, but the solution that I got to work best was this one. If I can figure out how to call that every time the list is redisplayed, we'll be all good on that front!

[n1474335]

There is a listener for oplistcreate events and its main handler is here: https://github.com/gchq/CyberChef/blob/master/src/web/OperationsWaiter.js#L150

[n1474335]

Thanks very much for this. I've made some further modifications to fix some edge cases but I think it gives the correct behaviour now.