Add "on the fly" addition of donation sites (we have it for product drives, and manufacturers) when entering a donation

[cielf]

Summary

Add the ability to add donation sites "on the fly"

Why?

Consistency. We have it for product drives and manufacturers (who aren't going to added any more often). Would also allow us to take the optional "donation sites" off the getting started

Details

On the new donation page, reached by clicking "Donations", then "New Donation" --
When you are entering a new donation (or editing one), and you select "Donation Site" as the source, you should be able to choose "Add a new Donation Site", instead of just the current donation sites.
This would bring up a form to fill in with the donation site's info. When saved, that donation site would be populated into the donation site field and appear in the drop-down for donation sites.

This should work very much the same as adding a product drive at the time you are entering a donation.

Criteria for completion

• behaviour as described above
• tests to support said behaviour
• update the user guide to include the behaviour