epi v2 finalization

[munnsmunns]

Implements requested changes from the most recent Epi group feedback

• Change "Create" button to "Add Row"
• Add Hair to biomonitoring choices
• Add "Units" field to Data extraction
• Make Source, Age category, Exposure measurement types required
• Make Sex, Type of variance estimate (assuming variance is not filled out), Lower/upper interval type (assuming lower/upper interval is not filled out), Lower/upper bound type (assuming lower/upper bound is not filled out) not required
• double length of AdjustmentFactor description field
• Add a symbols dropdown to forms for easy copy/pasting of greek symbols, super/subscripts, etc.
• Re-order Outcome fields and rename to System, Effect, Effect detail, and Endpoint/outcome
• Central tendency displays median or mean if available, does not show range in the table view

Remove the feature flag, clean up migrations, and release!

[shapiromatron]

Since this will be our release, can we also tear out the "coming soon" stuff that we previously had:

1. Remove the ENABLE_EPI_V2 feature flag
2. Update the Study detail page to allow for epi v2 extractions

[shapiromatron]

Nice job @munnsmunns and @casey1173! Excited to release this!

[shapiromatron]

changed the variable to NA instead of NONE, since those are two separate things

[shapiromatron]

this will stop propagation of all dropdown-menus on the page; I made it more narrow

[shapiromatron]

removed this span; it's not used anywhere. Also a span is an inline container, but a div is a block; which generally you'd only nest within other block elements, I think. My html5 knowledge is not great, but I think that's generally correct...