Enable filter date fields by default when $g_use_javascript = OFF

With javascript disabled, when ticking 'use date filters' checkbox in
view issues page, the associated selection fields to pick the date
criteria remain disabled, preventing date filtering.

The code now detects the state of $g_use_javascript and sets the
'disabled' state of the date selection fields accordingly.

Also removes 'Undefined variables' system notices.

Fixes 0015255