various admin table cleanups

The path-admin assignrole table looks better at 100% width, and it makes it more responsive.

setting things to left-align is mostly a holdover from when text-align:center was used to center things.

The rule .path-admin #assignrole .admintable is broken, it should have
been #assignrole.admintable without the space, just deleted the rule
as clearly no-one missed it.

.roleassign table micro-management not needed (though removing the
generaltable class from this layout table would help, I've got a bug filed about
that somewhere, ah, https://tracker.moodle.org/browse/MDL-41361)

Not sure what's going on with the hidden and then shown again legends. Certainly bad
HTML whatever the case, so lets see what happens if we delete this CSS.

added a couple of bug numbers for things that can be removed
once the bug is fixed (one is already going into integration).

Made the current theme highlight in the theme chooser look nicer.

Removed some RTL versions of stuff that is currently commented out (which should itself probably be deleted).

Cache table stuff was already 100% wide with Bootstrap styles, so this was redundant.