Dashboard data counts api, models, wiring

[michaelbrooks]

@geosoco and/or @katiek, please review. Here are the highlights:

This includes the following brand new items:

• data counts API controller
• data counts model
• library of date utilities
• new base model class

It includes the following changes of lesser significance:

• additions to the data counts API doc
• common/models/data_counts.js filled in, with fetch status tracking (loaded)
• dashboard/views/data_summary_view.js filled in, using spinner during data fetch

You can see it in action at the test url https://depts.washington.edu/etcgroup/test/dashboard (at least until I deploy over it :)

[geosoco]

overall, generally looks good.