Account form creates duplicate subscribers

It's all very baffling - layer upon layer of nested callbacks! Here is a patch that seems to solve it, but no idea what else it might break.

The test failures are just the same as in the base branch, so effectively this has passed the tests.

Here is some further explanation. There are three submit handlers, called in the following order:

1. '::submitForm'
2. '::save': creates the subscriber entity.
3. '::submitUpdate': writes the subscription records.

With the checked-in code, submitForm calls submitUpdate, which writes the subscription records, auto-creating the subscription entity. Then step 2 save creates another entity.

@Berdir please can you take a look when you have a moment. Only a one line fix, but it's very subtle so I don't want to commit without expert confirmation.

I just requeued the tests.

Thanks @Berdir.

Yes I have already been thinking about follow-on actions to fix the duplicates and prevent future ones - see #2937251: Prevent duplicate subscribers.

I propose that we should prioritise fixing this issue for now as it's a one-line change and at least stops making the problem worse. I will add a test as you suggest.

The other issue is more complex with schema changes and update hooks so it will take more work.

@Berdir tests done

I propose that I tag all issues that I plan to commit after waiting 7 days like this "Plan to commit". It should make it easy for you to find/check them. If you want to indicate that you have checked it, then you could remove the tag and set to RTBC in which case I don't need to wait.