Don't allow fileEditing to be disabled if imported files exist in group

[dstillman]

Return 409, which website can check for. The website will then need to prompt to delete all files in the group, perform a search for itemType=attachment, review the items' link modes (or use a linkMode search param that doesn't currently exist), and issue batches of DELETE requests before changing the setting.

/cc @fcheslack

[dstillman]

OK, new plan: the client should just deal with this better, because it's reasonable to want to amass some files and then turn off this setting, and the client has to deal with it anyway because the setting can change any time (or change to 'admins' from 'members') after files have been added locally.

For metadata, client can use pristine JSON to revert changes, or delete if not previously synced. Not sure about for files.

[dstillman]

When we decided against this, it seems like we were only focused on the client losing write access and didn't consider that existing files become inaccessible and still count against storage. So we should still do this after zotero/web-components#48 is implemented (at which point it will be unnecessary, but it will still be a useful sanity check).