Remove dashR dependency on htmltools #54

Given that we are aiming for parity with Dash for Python, and that "rolling our own" dependency handling/resolution logic is a minor hurdle, it seems prudent to eliminate references to code from htmltools in dashR.

Most of the references to functions in htmltools have been excised, with the exception of

Aside from removing references to this package, I'll need to write a function which has the same effect as htmltools::resolveDependencies.