Calculate last action date on OCD Bill and Event save

[hancush]

Description

This pull request extends the post-save signals for OCD bills and events to also calculate and store the last action date for a bill. This logic was previously defined on the Metro bill model, however it will be useful across jurisdictions.

Note: We refresh the last action date on event save because event agenda items can influence the value of last action date.

Related issues

Connects opencivicdata/scrapers-us-municipal#300, Metro-Records/la-metro-councilmatic#541.

Testing instructions

• Check out this branch of django-councilmatic.
• If you haven't already, clone LA Metro: git clone https://github.com/datamade/la-metro-councilmatic.git and cd into the project directory.
• Switch to the local Councilmatic branch: git checkout spike/hec/local-django-councilmatic. Update the volume mount in the scraper service to point at your local django-councilmatic directory.
• Build the scraper service: docker-compose build --no-cache scrapers
• Run a bill and event scrape: docker-compose run --rm scrapers sh -c "pupa update lametro bills window=7 --rpm=0 && pupa update lametro events window=14 --rpm=0". Confirm that they succeed.
• Shell into your Councilmatic database and confirm that some bills have values for last_action_date:

  docker-compose run --rm postgres psql -U postgres -d lametro
  postgres=# select count(*) from councilmatic_core_bill where last_action_date is not null;

[fgregg]

looks good.