Improve accessibility of forums flagging feature

The "Report Misuse" div is now focusable, responds to keyboard input,
and has appropriate attributes to indicate that it is a toggle button.
This fixes FOR-201 and FOR-209.
Increase color contrast for forum features

The icons and text for pinning and flagging ("Report Misuse") had
contrast ratios that did not meet the minimum WCAG requirement of 4.5.
This fixes FOR-200.
Merge pull request #1265 from edx/gprice/forum-flag-a11y

Improve accessibility of forums flagging feature