luci-base: introduce new LuCI.ui.Table class

Introduce a new `LuCI.ui.Table` class to simplify generating HTML tables
for data output and turn the existing `cbi_update_table()` procedure into
a wrapper around this new facility.

Tables generated by the new class closely resemble the old markup but
provide additional sorting capabilities.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>