Issue #3513678 by codebymikey: Ensure the initialDate parameter is configurable

This is because the "start" query string might be used for something else.

Default behaviour is to fall back to the "startParam" for consistency and backwards compatibility purposes since that's the start of the event feed.

However there's a potential usecase where the start date and initial date can be different, so having it available under a different configuration makes sense.

Closes #3513678