This creates an overriding `rswag:specs:swaggerize` rake task that also
adds plugin paths, and updates spec helpers to handle plugin paths.
Also adds the spec files for the discourse-calendar events index
endpoint.
### Testing
Running `rake rswag:specs:swaggerize` now generates the same
`openapi/openapi.yaml` file, with `/discourse-post-event/events.ics` and
`/discourse-post-event/events.json` GET documentation.