Make bugsahoy accessible (ARIA, WAI, etc.)

[jdm]

It's worth getting in contact with David Bolter (davidb on IRC) to get solid pointers on accessibility resources.

[jdm]

Keyboard navigation is pretty reliable right now. Remaining issues:

• The tooltip doesn't get read out if it's not an alert (but an alert overrides other text, unfortunately)
• There's no indication that loading is taking place (the throbber alt text isn't read for some reason)
• There are enough checkboxes that it might be worth adding an accesskey for the results
• Need to check if the extra information boxes are read out