remove need for :not() btn selector

removal of the need for
.btn:not([class*='btn-'])
by incorporating the 'not' styles into basic buttons, and then modifying the btn- classes to overwrite the base button styles.

i was able to get the library down to 1k gzipped by doing this (though I still haven't brought over all the navbar styling, so it's probably a bit more than that)

this means we can keep btn-small (which i've also renamed to btn-sm) to be consistent with the other class names.