Vis utils

[arik-shurygin]

this PR adds the following functionality

1. Implements default saving of visualizations along with all calls to abstract_azure_runner.save_inference_posteriors() via the save_mcmc_chains_plot and save_correlation_pairs_plot functions
2. Adds the vis_utils.py file to src/ that declares some useful utils functions for generating views of the model outputs (both the azure_visualizer_timelines.csv and the checkpoint.json files)
3. adds plotly and shinywidgets back into our version control since we are still using shiny to a limited extent

The idea behind these changes is to further automate the visualization pipeline so users can more easily see the results of their fits and/or projections.

This has the added benefit of opening the door to faster load times in the shiny_visualizers/ apps as they can now load static images right away (especially useful for large plots like the correlation pairs plot.

CLOSES #276

[mrbatist]

There are a lot of changes, but it looks good to me from a quick read through.