Move conditional milestone filtering to API

[hannahkates]

Currently a lot of data handling, recoding, and filtering for milestones is happening in the front-end (here: https://github.com/NYCPlanning/labs-zap-search/blob/develop/app/components/project-milestone.js). To simplify future updates and better separate concerns, some of this functionality should be moved to the API instead.

Things to move:

• [x] Display name recoding
• [x] Date handling
• [x] Filtering which milestone dates to include based on project status (only filed or further along)
• [x] Milestones descriptions based on whether project is ULURP or not

[hannahkates]

Issues noted during implementation:

• "Filed" projects aren't labeled correctly in the 'dcp_publicstatus' field. Several of them have actually advanced into the public review process - these are showing up on the production site when they shouldn't be:
  • 2019Q0292
  • P2018K0108
  • 2019K0287
• Several "Filed" projects are over a decade old and listed below them are a bunch of projects that don't provide a date for sorting.