luci-base: restore titles class

luci-base: restore titles class

follow-up to 720c96ce5b85583c8c6db6ef31857dc8a3eeb600

While ::before CSS styling has been deactivated, and
'content' removed, cbi-value-first-field is a bit
ambiguous. So, restore cbi-section-table-titles, and
replace the ::before tags in CSS.

Remove also the data-title for title rows. We already
insert titles for named headers and rows via HTML and
not via CSS after the mentioned commit.

Signed-off-by: Paul Donald <redacted>