MDL-39824 theme_bootstrapbase and theme_clean: Simplify two column layout

Hi Sam,

You mentioned in the 'tracker' that 'Aha, yes the grid system changes. Again a downside to this whole change set when looking a the two columns layout and switching block positions from left to right. I worked around this here: https://github.com/samhemelryk/moodle/compare/master...wip-MDL-39824-m26#L21R28
That uses the bootstrap mixin used in defining the grids to adjust the column layout in the circumstance side-post is being used there.' and that 'Because the switch is defined we must ensure that side-pre and side-post both exist there they'll still be switched even though there is only a single block region.  I've not thought up a good solution to this yet, but hopefully there is one out there.'

I think this solves the solution as it:
Eliminates the nesting.
Removes the extra grid system changes.
Works for LTR and RTL.
Copes with 'side-post' and no 'side-pre'.

I've put extra comments in the 'columns2.php' file to explain the code which you may wish to remove.

Cheers,

Gareth