ui: Move the background color of the table headers to a class

ui: Move the background color of the table headers to a class

As we are using "sticky headers", the column headers stay apparent when
scrolling down, we need to specify a white background to hide what's
underneath.

With the addition of Seres, the patchlist table is not the only table we
want this behaviour on, so let's make it a CSS class rather then a
selector on the patchlist id.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Acked-by: Stephen Finucane <stephen.finucane@intel.com>