Remove dashR dependency on htmltools

[rpkyle]

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

https://github.com/plotly/dashR/blob/7fa2581d41e8a33c8966490f51ef0ccf371edfd8/R/dash.R#L554 https://github.com/plotly/dashR/blob/7fa2581d41e8a33c8966490f51ef0ccf371edfd8/R/imports.R#L6 https://github.com/plotly/dashR/blob/7fa2581d41e8a33c8966490f51ef0ccf371edfd8/R/css.R#L33 https://github.com/plotly/dashR/blob/7fa2581d41e8a33c8966490f51ef0ccf371edfd8/R/dash.R#L71 https://github.com/plotly/dashR/blob/7fa2581d41e8a33c8966490f51ef0ccf371edfd8/R/dash.R#L361

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