Improve accessibility of forums navigation elements

The home icon and discussion topic drop-down icon now have text that is
visible only to screen readers. The visual change between the icon and the
topic name with down-arrow is now achieved with the CSS visible property
instead of the opacity property, so only one at a time will be visible to
screen readers as well as visually.
Remove some cruft from forums SASS
Merge pull request #921 from edx/gprice/forum-nav-accessibility

Improve forums nav accessibility