Improve HTML representation

[jessegrabowski]

1. This should be shown when you just look at the model, without having to call a separate function on the GCN file
2. It should include summary information about the model, as well as information about the raw blocks -- final list of variables (with name/description after Add support for Latex printing of models  #16 ), priors, equations, provided steady-state (if any).
3. Consider displaying the minimization problem described by the blocks in a more mathematical way. That is, instead of showing headings for objective/controls, show $\min{\text{controls}} \text{objective}$. Could also consider $s.t. \text{constraints}$, but I think it would quickly get too big. At least try it.