enhancements to plot_forecast

[vpnagraj]

@stephenturner i'm assigning this to you. let me know if it looks good.

a few edits to plot_forecast() related to issues we've had with the shiny app development and performance ... but also generally useful i think:

• facet labels now have location names
• now using consistent y axis scale and big mark ,
• plot_forecast() now excludes cumulative case panels

those are being tracked in https://github.com/signaturescience/focustools/issues/45 and #39

[vpnagraj]

btw when we merge lets leave the shiny-app branch open

[vpnagraj]

good call. added that "by" arg. mergin