Refactor re-index program endpoint

[joneubank]

The existing endpoint that allows users to request a program to be reindexed needs a few modifications to work on teh RDPC instead of in the DCC.

The endpoint is currently defined in src/index.ts: https://github.com/icgc-argo/donor-submission-aggregator/blob/develop/src/index.ts#L24

Changes needed:

1. Move endpoint out of server definition file and into a dedicated router file
2. Add authorization rules to this endpoint to only allow it to be accessed by RDPC Admin with Write permissions
3. Wrap existing index endpoint in a feature flag. This flag will be enabled in the central platform environment so that the existing system can trigger a reindex. It will be disabled in the RDPC environment (default behaviour) to prevent access to this endpoint that does not enforce authorization rules.