Load jQuery on the front end for logged-in users

[azaozz]

This will let us simplify the toolbar JS and improve user experience: hoverIntent, additions by plugins that require jQuery, etc.

jQuery can be loaded in two different ways:

• Directly, by enqueueing it with script_loader and then adding wpjq = jQuery.noConflict(true); where true means other version of jQuery will be left with the globals.

• By outputting some JS at the very end of 'wp_footer'. This JS would be able to check whether jQuery is already loaded and if not, load it by appending a <script src="..."> tag to the head (note that this method would load jQuery asynchronously).

Previous discussion in some of the comments on #18758.