Convert mediawiki.toc and mediawiki.user to using mw.cookie

Convert mediawiki.toc and mediawiki.user to using mw.cookie

* Remove redundant 'path' parameter (handled by mw.cookie)
* Remove redundant 'expires' parameter (handled by mw.cookie)
* Return value for absent cookie is now reliably 'null'.

This changes the cookie name due to mw.cookie adding the standard
cookie prefix. This will cause existing values to be lost. Make
use of this oppertunity to rename some cookie names.

* mw_hidetoc -> {wikiprefix} hidetoc
* mediaWiki.user.sessionId -> {wikiprefix} mwuser-sessionId
* mediaWiki.user.bucket -> {wikiprefix} mwuser-bucket

This is a re-submission of a4d3d3b427713, which was reverted due
to T101857. Commit amended to use "sessionId" instead of "session".

Bug: T67384
Change-Id: Ibe88778cf3b6db90b3875c89305ffba53ac84104