Repeatedly check if payment pointer in monetization tag has changed

[vezwork]

1. Repeatedly check if monetization tag payment pointer has changed. setinterval can be used. Could be checked every ~second.
2. Save whether payment pointer has been validated recently. i.e. within the past hour.
3. Validate payment pointers which have never been validated, or which have not been validated recently.

This data (and potentially also the logic) would fit well in the AkitaPaymentPointerData class.

[vezwork]

Related discussion: #14 (comment)

[vezwork]

Will fix the following problem:

• If a user who is not using a payment provider visits twitch.tv, it will never be registered as monetized because sites are only registered when the page loads. Twitch.tv adds the monetization tag programmatically after the page loads so it will never be registered.