Layers list view

[mradamcox]

Adds new quality measure fields to layer schema in API, and creates a new Layers list view that uses a new paginated /layers2 endpoint. (To be cleaned up and turned into /layers eventually, but there is an existing /layers endpoint that is not paginated and requires a per-map filter. Ultimately that will be replaced by the new one, but there is a lot of other API clean up to do at the same time.)

See discussion #420 for more info. The terminology and normalization ideas I proposed there have been implemented, and @tannewt had some further suggestions for how to visualize these errors in a list setting that we should explore soon.

For now, this page is for staff users but soon will be public. This layers list component will ultimately be added to a few other places as well:

• Map overview (all layers in a map)
• Profile (all layers a user has georeferenced)