TPAC: Simplify opac-button styling

TPAC: Simplify opac-button styling

Conflicting padding declarations in the stylesheet, in an old attempt to
balance out the sizes of various INPUT types vs A elements in Chrome vs.
Firefox, were resulting in increasingly odd sizes for the buttons
(depending on which browser you used). Switching to a BUTTON instead of
an A tag and dropping all of the padding declarations improves the
situation on Firefox, Chrome, and IE8, while simplifying the CSS.

Signed-off-by: Dan Scott <dscott@laurentian.ca>