Consider simplifying Bootstrap navbar default, inverse and calcite-dark

We might want to re-think and simplify this. Right now it's pretty confusing to have a completely different Sass variable set and a completely different build output just for calcite dark, especially given that bootstrap already has navbar inverse. Bootstrap inverse gets us 50% of the way towards calcite dark.

Bootstrap Navbar Default = Calcite Default
Bootstrap Navbar Inverse = (similar but not the same as Calcite Dark, Calcite Dark styles more elements)

One thought is to just use navbar inverse and provide overrides for the remaining calcite dark elements down the rest of the hierarchy. It will add a few extra lines of CSS but it will prevent us from having two separate builds.

Thoughts? @CassidyB @nikolaswise @TheBlueDog

This is related to #182.

No files, tests, or entry points are named. Start by reviewing the Bootstrap navbar theme variables and the separate Calcite Dark build output, then compare how navbar inverse and Calcite Dark style the remaining hierarchy. Done means agreeing on whether one build with overrides can replace the separate dark variable set and build.

Written by the indexing model from the issue text.