Start the switch to BEM

This is inspired by https://github.com/ivaynberg/select2/issues/2826
and starts the general transition to using selectors with a lower
specificity. This should make it considerably easier to override
the styles in the long run.

Unfortunately libsass does not support the SASS 3.3 style for BEM
where we can just use `&__m` for defining sub-modules. For now we
are still using a limited amount of nesting, but nowhere near as
much as we did in the past.
Switch the multiple selection container

This switches the container to use BEM for the class naming.
Finished switching main container to BEM
Finish the core switch to BEM

This switches all of the dropdown components to use BEM.
Converted classic theme to BEM

This should be the last of the changes.