make collapsibleTabs more resilient to CSS changes in Vector skin

Using .offset() instead of .position() will keep this working
if #left-navigation is changed to be positioned with margins and floats
(like #right-navigation currently is) instead of absolute positioning.

No functional changes right now, but this goes with commit Iedaebefc
in core, which does the above CSS change.

Change-Id: Ie34dce9a9919976bfab8f1235b7cbe3a23f784df
(bug 44385) move jquery.collapsibleTabs module from MediaWiki core

It was entirely Vector-extension-specific (so very unlikely to have
been used by anything external), deeply interconnected with the
extension (its internal functions were overwritten by a script from
it, making it even more incompatible with other uses), and this
functionality (being really a partial workaround for low-res issues
with the skin) belongs to the extension instead of the skin anyway.

Corresponding MediaWiki core change: I7c0f42d92

Change-Id: Ic6b622279d3502d522128da14ba2346c84318325
Merge changes Ic6b62227,Ie34dce9a

* changes:
  (bug 44385) move jquery.collapsibleTabs module from MediaWiki core
  make collapsibleTabs more resilient to CSS changes in Vector skin