Add PostingDataAdmin

[gregjkal]

Add an admin page for the PostingData model for better data visibility.

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Testing Notes
This is a simple addition of an admin page for an existing model.

• You should see a new admin section under MAILING LIST (note also that I've fixed the name - previously MAILING_LIST) called Posting data
• If you click on it, you should see a list of mailing list posts, with a timestamp and a name
• Clicking on individual items brings you to a detail page with the same, editable information
• Note I've also fixed the verbose_name_plural for the EmailData and SubscriptionData models. They now show in the admin as Email data and Subscription data without an "s" at the end, as you'll see on stage/prod.

[daveoconnor]

LGTM