new subtask: nyc_intersections crashcount

[gregallensworth]

This builds on PR 3 (some minor fixes) so merging PR 3 may resolve a few diffs and make this one easier to read.

DO NOT MERGE THIS until some decisions are made re https://github.com/GreenInfo-Network/nyc-crash-mapper-chart-view/issues/73 as our direction may change as we discuss.

• adds new utility function get_date_monthsago_from_carto() to get a PostgreSQL date string from X months ago
• adds new subtask function update_intersections_crashcount() to update the new nyc_intersections table crashcount field This is per https://github.com/GreenInfo-Network/nyc-crash-mapper-chart-view/issues/73 in which we will want to display intersections and compare them

[gregallensworth]

Closing this one, in favor of a new PR. There have been other cleanups and changes since this was created, and the new PR will be much easier on the eyes and brain