simplenews_issue field type should not store subscribers

Small progress and new starting point :). Need to work on this in way to provide update config function.

Created update config function. Tried with update database update function but did not succeed and could not provide useful patch for that (will need some instructions about that)

Made some progress, thanks to @berdir and his instructions and tutorial. Currently after debugging and running update function i got an error ALTER TABLE {node__simplenews_issue} CHANGE `simplenews_issue_subscribers` `` DEFAULT NULL;. Still a little more work on this, but i think this patch will be good starting point IMHO :-)

Small fix, hope test should pass now...

Setting to needs review, re-ran multiple times the test on local machine with applied path and there is no single failure. Seems random test fails...

It seems we have a random fail here:

testSendNowCron
fail: [Other] Line 340 of modules/simplenews/src/Tests/SimplenewsSendTest.php:
Mail has correct subject
Value '[Default newsletter] wjBX]>&d&d'.

So it seems that there is a random fail if the newsletter random string contains HTML brackets lt, gt. Let's investigate in a follow-up. To start, change testSendNowCron() line 281 to the value above and reproduce the bug.

I found two instances where the subscribers value is still used.

1) SendStatus::getMessage Maybe can just remove the ?: and always call simplenews_count_subscriptions?

2) IssueItem::setValue Can just delete this code I think.

Re #14 First item is solved by latest patch in #3030432: Simplenews list page confusing/wrong "send status"

After thinking about it I think we should keep subscribers field = "number configured to send" because I see a valid distinction from the sent field = "number actually sent". Even when a newsletter issue has completed sending the two might be different because of skipped emails or errors that exceeded retry #3052714: Send failures retried forever. Let's not take time deleting something that has a little value!