move rtl only CSS to their own files

And simplified some of the remaining RTL rules in the less.

Also added a switching mechanism based on the current language,
note that this will then cache RTL or LTR until the next theme
change so it's not suitable for sites using both directions.

There's still an issue with the columns on the front page, I
think there's some Moodle mechanism for RTL that's fiddling with
the block order and interfering with the basic Bootstrap grid. Will investigate that next.

But other pages, and in responsive mode it seems to be mostly working fine (install an RTL language, turn on theme developer mode and switch language to test, note that the moodle-rtl is a one off, it won't be autogenerated on changes.... yet).