Move unit links' style from base.css to typing.css

The style for #unitLinks is currently specified in base.css.
However base.css is meant to contain styles for layout elements
only. The #unitLinks element is a typing tutor's element.
Therefore, move it to typing.css.
Add tooltips to feedback pane

Add helpful tooltips to the following elements in the feedback
pane:
  1. Progress bar and progress percent
  2. Restart link
  3. Speed displayed in WPM
  4. Error rate

Do not display the speed in CPM since this would go as a tooltip
for the speed displayed in WPM.