Fix bug 939112: Newsletter form choices only supported.

The newsletter forms will now only display choices
for the languages supported by the specific newsletter.

Removes the newsletter middleware and moves form processing
into the template helper function.

Also fix bug 914680: Enable language choice for hacks.
Merge pull request #1419 from pmclanahan/newsletters-show-only-available-languages-939112

Fix bug 939112: Newsletter form choices only supported.