Add project, region, and site statistics routes

[atruskie]

Add descriptive statistics to endpoints to replace the blurb that's currently in the rails views:

Should include:

• the first and last date of recordings in said object
• total duration of recordings
• density of recordings
• a distribution map (density per some time unit, across the interval of captured recordings, ?maybe per week?)
• annotations count
• download/view stats of recordings

These stats should be added to:

• projects
• regions
• sites

Any other stats?