luci-base: form.js: add cfgvalue(), formvalue() and getUIElement() helpers

This commit introduces new per-section cfgvalue(), formvalue() and
getUIElement() helper functions which complement the respective per-option
functions.

Their intent is to simplify querying input data or obtaining UI widget
instances from other options within the same section.

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