Add MapFeature model

* Superclass for Plot (and, in the future, RainBarrel etc.)
* Has feature_type field, so generic searches can just query MapFeature rather than Plot, RainBarrel, etc.

Make temporary fields
* Make an explicit "Plot.mapfeature_ptr" field for migrations to use (it will become implicit once Plot inherits from MapFeature)
* Make a temporary "Tree.mapfeature" field for migrations to use (it will become Tree.plot)

Migrations
* Create a MapFeature for every Plot
* Make Plots point to their MapFeature
* Make Trees point to their Plot's MapFeature
Finalize Plot and Tree classes

* Plot inherits from MapFeature
* Delete Plot fields moved to MapFeature
* Tree.plot now points to MapFeature instead of Plot
Add safety code in plotMarker.js