Add the ability to disable newsletters

[jenlampton]

I have an older site, upgraded from Drupal 7, that has 28 newsletters. Many of these are for events that happen 2-3 times a year, and after the event is over, the newsletter is not used anymore.

It would be nice if we could retain the old newsletters, sending data, and subscription information, but mark these newsletters as "Disabled".

A disabled newsletter would longer appear in the list of options when creating new Newsletter Issues. For older issues that were part of a newsletters that is disabled, people would be unable to change the newsletter selected.

[alanmels]

Nice idea, but since the #70 is broken, maybe you could file an independent PR for this feature not tying it to the changes in the other PR?

[jenlampton]

Done, rebased, resolved conflicts, and renumbered the update to simplenews_update_1005.