MDL-43867 Atto: Accessibility improvements.

1/ Set the aria-labelledby attribute on the contenteditable div (find the label from
original textarea)

2/ Store/restore the selection for the contenteditable div when it is focused. This allows
you to select some text, then go to the toolbar and click a button, and the selection
will be restored before the button effect is applied.

3/ Add an accessibility helper plugin.

From testing in all screenreaders, I found that all of their support for contenteditable is not great.
They treat it like a textbox - which means you can type and edit text, but it tells you nothing about
the styles, links or images in the editor. So I added a button to the toolbar, that is only accessible
when navigating via keyboard, that opens an accesssibility helper dialogue. The dialogue shows the list
of current styles, a global list of all links, and a global list of all images. Choosing an image or link
from here, will focus on the editable region, and select the link/image.

4/ Add an accessibility checker plugin to Atto.

Checks for images with no alt, images and links with filenames as alternate text/link text, and contrast ratios
less than WCAG 2.0 AA.