IdM Accessibility improvements

* Adds an accesskey to login/profile link for quick use
* Changes the list heading to use the wdn-text-hidden class (screen
reader only)
Add helper to allow screen-reader only text [A11Y]

Closes #622
Remove wdn-text-hidden from apps CSS.
Unified search does not need to submit the submit button

Removes the submit input's name
Appease FAE by putting a list descriptor above the social outposts
Improve [A11Y] for directory search

Removes fieldset, autocomplete, and adds a submit button (hidden)
Adds an accesskey to the apps drawer to match the 2006 template shortcut
Ensure the unified search label matches the size of the other headers
Remove navigation CSS that overrides wdn-text-hidden helper