Allow users to access their purchase info using only their purchase email

• When a user visits EDD's Purchase History page (e.g. /checkout/purchase-history/), they will see this screen:

• After entering an email, a verification link will be emailed.
• The verification link goes back to the Purchase History page, but with the user's purchase information visible.

• This add-on creates a custom database table, named eddnl_tokens
• A fake user, eddnl is created with a strong password and no permissions. This fake user is needed to bypass many of the is_user_logged_in checks within EDD and its add-ons.
• The edd_payment_user_id filter is used to set the correct user when viewing payment information.

eddnl_verify_throttle

Time (seconds) between when token requests can be sent to a given email address (default: 300)

eddnl_token_expiration

Time (seconds) before an access token expires (default: 86400)