Add MapFeature model, superclass for Plot (and, in the future, RainBarrel etc.) 

* Plot uses Django multi-table inheritance. That means the Plot Python class has all the same fields it used to, but they're stored in two DB tables (treemap_plot and treemap_mapfeature).
* Migrate existing Plot data to updated Plot and MapFeature tables
* Adjust some subtle low-level things that depend on DB structure. (No changes to higher-level code dealing with Plot objects.)

Notes
* Migrating the plots is slow (about 3000 plots/minute on my machine)
* There is currently no reverse migration, but we can write one if we need it
* Small updates to OTM-Tiler coming soon