(bug 44385) make ext.vector.collapsibleTabs actually work

It didn't hide empty dropdown since the headings having been changed
from h5 to h3 some time ago. I guess nobody noticed because this would
only ever be visible to non-autoconfirmed users who don't have the
right to move pages (as it's the only situation where the dropdown
could contain exactly one item, the history link).

Additionally, it was using different element dimensions than the skin
itself, causing the dropdown to be slightly borked, and different
animation durations for hiding and showing. Nobody noticed since it
didn't work at all, as explained above.

Also applied some generic cleanup and code conventions.

Change-Id: Ib4ffef1b8868cbad3d4597a95c9468565c0c8849