feat(ApiWeb, config): add StopEventsController and resources behind feature flags

[runkelcorey]

Summary of changes

Asana Ticket: 📸↩︎ construct database query (ApiWeb.StopEventController + ApiWeb.StopEventView.location + router)

1. Adds feature flag for stop events modules in ApiWeb. These are true in dev and test but false in prod and by default. These allow the endpoint to be hidden and prevent the relevant modules in ApiWeb from being compiled (or invoked) in production
2. Adds stop event endpoints to router resources
3. Adds StopEventController
4. Adds non-raising commands to look for stop events env vars

How were these changes validated?

1. Lots of tests for the controller, as per usual
2. Ran mix phx.server locally withstop_events_route true and false, observing that the paths were not shown on the docs when the flag was false and were shown when the flag was true

What questions should reviewers consider?

1. Is there a better way to do these feature flags? I don't like that I need to set the entire module within a conditional but, in my limited attempts, it seemed more annoying to specify the conditionals on a per-function or per-test basis

[runkelcorey]

just wrapping this in a conditional; otherwise, no change

[runkelcorey]

just wrapping this in a conditional; otherwise, no change