Move base into ownmacro/slot, make other urls relative

There is now only one absolute url left. Unfortunately base tags
cannot take relative paths. We can hack this using JS.
Stop double-wrapping popover helpers
Compile editor to dom object early, expose for modification
Cheat sheet plugin

Contains small changes in math plugin to expose the editor dom, so the
dom can be modified. The dom is cloned for each editor instance, including
event handlers and data, so you can bind event handlers once to this
template. Also includes the help panel.
Use the cheatsheet plugin for oerpub.
Refix auto-resizing

pre needs to use same font size as the textarea in order to resize properly.
This was lost when testing was dumped.
Merge branch 'oerpub-ticket414' into testing