Create endpoints to get counts

[tkoenigusgs]

We need to be able to pull counts of these items from the database:

• planned HWMs

• flagged HWMs

• surveyed HWMs

• approved / unapproved HWMs

• deployed Sensors

• recovered Sensors

• uploaded Sensors

• approved Sensors

We also need to filter those counts by sensor type (storm-tide / wave / RDG / precip) or by State. So rather than creating a huge mess of new endpoints for counting, I'm thinking the easiest thing might be to add a "counts_only=true" attribute to the FilteredHWMs and FilteredSensors endpoints. Then I can use all the existing filtering logic of those two endpoints, rather than creating a bunch of new counting endpoints.

[tkoenigusgs]

This issue may be replaced by FEV issue #411, or at least see issue 411 for a more updated list of endpoints that need counts.

[HansVraga]

USGS-WiM/FEV#207 ☝️ FEV issue Todd is referring to