luci-base: enable table filtering via .filterrow

luci-base: enable table filtering via .filterrow

filter() is already available to pre-filter any data we
want to display in a table prior to render, although this
is primarily to filter on data types in a config file.

Now it is possible to display a header row (thead)
with text fields to search a table column. Set the table
property .filterrow to true to enable. It is null by default
so the filter header row is disabled. The filters work
cumulatively, so they can be used in combination.

Stabilize column widths so when nothing shows due to
filters, the filter header rows don't eat up all the table
width and jump around if action buttons are present at the
right end of data rows.

Signed-off-by: Paul Donald <redacted>