Automatic filtering down to only shared states should be togglable

"Total across shared states" behavior is confusing. The alert that plots are only showing should be WAY higher on the page and brought to the user's attention (red, bolded, etc).

The user should be able to turn on/off "Total across shared states" behavior to throw away states not available at all time points.

Related: Nick Reich has previously requested that the state filter logic be changed to not be so strict. At the moment, we throw out a state even if it's only missing for one week.

Start by locating the implementation of “Total across shared states,” the state-filter logic, and the alert that says which states are shown. Trace how filtering affects the plots and how the alert is rendered. Done means users can toggle the shared-state behavior and the alert is made substantially more prominent.

Written by the indexing model from the issue text.