jquery.makeCollapsible: Enable passthru for links in premade toggle

Clicking on a regular link inside a premande toggle is supposed
to follow the link instead of collapsing/expanding the
appropriate element.

Broken during a rewrite in Id3f457a8 (2638b16c) - the comment
reflected the intended behavior, but the code was wrong.

Regression test included.

Bug: 46848
Change-Id: Id571f8d612e5131845d381121604c2b1116a077f
Merge "jquery.makeCollapsible: Enable passthru for links in premade toggle"