Refactor keypress activation for forum buttons

This removes the use of the space key to activate buttons, which is not
expected behavior.
Improve accessibility of inline discussions

The show/hide toggle now properly acts like a button, and focus is now
trapped on the loading element for both the show/hide action and loading
of a new page.
Merge pull request #1765 from edx/gprice/inline-discussion-a11y

Improve accessibility of inline discussions