Add route to pass ids of objects to be indexed

The route accepts a JSON array of object identifiers. It will add background jobs to do the indexing. 

The route is behind a new constraint which requires an API key.
There is a config file for API keys, and it is added to the deploy.
(hence future deploys may break until the file is added to the secrets
repos). Addresses DLTP-117

Note the changes to `protect_from_forgery`.
Missing top-level class documentation comment.