MathJax: Add a renderTex hook to render elements on demand

Example usage: $('#wikiPreview').renderTex();

This supersedes: https://gerrit.wikimedia.org/r/23073
Bug: 35478

Change-Id: I18e2f089d56e42b8e603ac45d726942bb478580b
MathJax: Remove support for wikEd and AjaxPreview

With the new renderTex hook of https://gerrit.wikimedia.org/r/61183
there is no need for hardcoded support for these 2 scripts. The
scripts in question should call the hook themselves.

PS2: Fix wikEd reference of jsHint
Supersedes: https://gerrit.wikimedia.org/r/23073

Bug: 35478
Change-Id: I9811832430e9ddd8a4c03f265d07529a76efc38a