Chapter permissions do not work

[amyjko]

Expected behavior

When someone is added as a chapter editor, they should be able to edit the chapter and save.

Actual behavior

• Create a book
• Create a chapter
• Create a second account
• Add the second account to the chapter
• As the second account, attempt to edit the chapter. Edits do not save.

Cause

The cause of this is that our check Firebase rules for chapter editing do not correctly check chapter editing permissions. The eligible list of chapter authors is stored on the chapter object in the list of chapters on the edition document, but the rules are checking the chapter document, which only has a text. The challenge here is that there's no easy way to write a rule that checks the corresponding chapter being edited, because we'd have to get the chapter number from its ID, by scanning the edition's chapters for the corresponding ID. It may be possible, but if not, we'll have to denormalize some data to make the rule writeable.