Update UI to work without prevalence data

[jaraco]

In response to #1791, the plan is to pivot to a self-service model where users are required to input prevalence data, similar to the current behavior of "override location-based data". Specifically:

remove automated prevalence and instead offer a simple interface for the user to specify prevalence on their own, with 3 or 4 strongly highlighted "low / medium / high / extreme" presets, plus the same "override" checkbox we have now.
Benefits:

1. maintains the relative-risk / harm reduction usefulness of the site, while still anchoring predictions to some vaguely reasonable values
2. communicates that this has become a tool for showing risk under different assumptions, as opposed to trying to estimate true risk
3. keeps the guts of the site unchanged and adds minimal new interface elements, so that we can get it done and keep it running with a plausible level of commitment.

This bug tracks that work and some of the things that need to take place.

• Replace shutdown alert with link to updated expectation (microCOVID remains but in a new form).
• Replace UI for location selection with the "override location data UI"
• Remove code that relies on data pipelines and turn down pipelines.
• Remove or recreate the video tutorial.

[jaraco]

Vince, would you be willing to take on the "turn down pipelines" aspect of this issue? I know you've already stopped any alerts. Is there anything more that you would recommend to be done w.r.t. the pipelines following the JHU shutdown?