Move availabity to separate columns

[jrobbins]

This should resolve #790.

In this PR:

• For missing values, use a blank cell rather than ---.
• Define four new columns for desktop availability on four browsers and add them to the list of default columns.
• Allow for a column definition to add a CSS class to the th and td cells in that column (but not its category cell)
• Make the new columns and existing WPT columns use a centered css class.
• Define a renderer function for the new columns that shows a gray or full-color icon. Remove the check or minus icon from the WPT cells.
• Reduce the vertical padding for the header cells to make the double header rows look better.

[jcscottiii]

LGTM with some nits.

Also, I filed #1000 to give the same color/grayscale treatment for availability on the feature detail page. Not for now. But just as a future issue.