Admin Notices should be able to be dismissed forever

Currently, the `LLMS_Admin_Notices::add_notice()` will redisplay any dismissed notice 7 days after it was dismissed. This default value of 7 days can be overridden with a custom value.

When we wish for something to be permanently dismissed, we add a large value (730 days or 2 years) to make it seem like it's dismissed forever, but this isn't actually what happens.

See related issue #1278

Ideally the `add_notice()` method should have an option to "dismiss_forever". When a notice is dismissed forever we should record that it has been permantently dismissed the `add_notice()` method should prevent the addition of the notice.

Start at LLMS_Admin_Notices::add_notice() and inspect how dismissal durations and dismissed notices are stored. Define the permanent-dismissal option there, then verify that a permanently dismissed notice is not added again; the issue names no test or file to run.

Written by the indexing model from the issue text.