Issue #61: Add a 'Subscription Page' option for newsletters.

[jenlampton]

Fixes #67
(and #61)

[alanmels]

Applying this PR to a fresh setup locally and enabling the module is giving:

Error
SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'page' cannot be null: INSERT INTO {simplenews_category} (`tid`, `format`, `priority`, `receipt`, `from_name`, `email_subject`, `from_address`, `hyperlinks`, `new_account`, `opt_inout`, `block`, `page`) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9, :db_insert_placeholder_10, :db_insert_placeholder_11); Array ( [:db_insert_placeholder_0] => 1 [:db_insert_placeholder_1] => plain [:db_insert_placeholder_2] => 0 [:db_insert_placeholder_3] => 0 [:db_insert_placeholder_4] => My Backdrop Site [:db_insert_placeholder_5] => [[simplenews-category:name]] [node:title] [:db_insert_placeholder_6] => noreply@ddev.site [:db_insert_placeholder_7] => 1 [:db_insert_placeholder_8] => none [:db_insert_placeholder_9] => double [:db_insert_placeholder_10] => 1 [:db_insert_placeholder_11] => )