Use Pi-hole.net donate page instead of direct link to Paypal

[PromoFaux]

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What does this PR aim to accomplish?:

Rationale behind this: If we, for whatever reason, decide to switch away from Paypal, then we no longer have to push an update to the web interface, we can just update directly on pi-hole.net

Also, as per referenced issue below, the donate page on pi-hole.net is a bit more multi-currency friendly than the direct paypal link

Closes #1246