Improve accessibility of the 'Single' and 'Manual' preview modes

Here's a patch that attempts to fix the first bullet point with some invisible text in the preview links.

The second bullet point is going to be trickier.

Here's the start of a patch that monkeypatches the CTools ajax command to replace the modal in order to adjust the focus on preview. It doesn't totally work yet, but it does get all the information we'll need to adjust the focus, and its run at a time that I think will allow adjusting it.

Ok, here's one that works enough to review! This puts the focus on the first focusable element within the preview (which is the "Add" button) for both the single and manual preview modes.

Hm. I've been testing this in an environment with Radix, but it seems to fail on vanilla Panopoly...

Here's a new version of the patch which is cleaned up, and works for me on both vanilla Panopoly and with Radix.

Apparently, the tricky stuff I was doing with spaces around the hidden, screenreader-only text was confusing screenreaders in testing. Here's a new patch that doesn't do that - the output should be the same for sighted users.

Here's a Travis build just to double check that all is good:

https://travis-ci.org/panopoly/panopoly/builds/397028809

Tests passed! Committed upstream :-)