Make Acquisition Order aware it is for a Document Reqeust during its creation

[JakobMiesner]

When navigating to a pending Document Request edit and creating an Order from it (see image below), the Order is not aware it is created for a Document Request during its indexing.
This becomes a problem when we want to index information from Document Request onto the Order.

One of those cases was implemented here.
The PR added the field document_request_waiting_time to the Order index, which requires the creation date of the related document request.

The problem happens as follows:

1. Document Request X exists with state PENDING and has no Order attached to it
2. Librarian goes into the edit page for Document Request X and presses "create new Order".
3. Librarian gets forwarded to the Order creation page.
4. Librarian finishes creating Order Y.
5. Order Y gets indexed and looks for a related Document Request. No Document Request is found, as the pid of Order Y was not yet added to Document Request X.
6. The pid of Order Y gets added to Document Request X.
7. Document Request X gets indexed.

This results in a state where Order Y has a related Document Request X, but the index of Order Y does nto reflect it.

In the PR mentioned above this problem was solved by reindexing the related order when a document request is indexed as can be seen here, but this is rather unclean.