Deprecate bs_bootstrap subtheme

Problem/Motivation

Remove bs_bootstrap subtheme so we have only one base theme - there is really no resources to support more then one framework for now and having two base themes are complicating things without any benefits for now.

Discussed with @berdir a bit and he agrees with this, the only thing he mentioned is to keep empty bs_bootstrap subtheme as empty skeleton deprecated theme for easier updating of existing custom projects. bs_bootstrap will be fully removed in bs_base 3.x.