Admin actions for Orders

[Xiphoseer]

We need to start up the admin actions that actually do stuff for us.

• [x] Rejecting a list of orders -> change status to rejected, send emails
• [x] Ordering a list of books -> change status to ordered, download CSV file, send emails
• [x] Marking books as arrived -> change status to arrived, send emails

For implementing this, as this is the actual workflow and needs to be precise in what effects it has, I propose the following workflow:

1. Create a new branch named feature/admin-action-order/<type> where <type> is the one you chose to implement.
2. Create a pull request for that branch, referencing this issue, so that it appears here.
3. Implement and commit to the pull request branch.
4. Wait for review and merge.
5. Edit this post to mark the checkboxes as checked by putting an [x] instead of [ ].

Additional required actions may be added at any time.

[Xiphoseer]

Close this issue, as the only thing left is Student Notification, which is considered in #17