Add payment form for donations

[kasugaijin]

The site would benefit from a simple donation form on the /donate route. Dicussion with the client concluded that Paypal is the most suitable payment processor to use as it is the one currently used by the client, and therefore requires little effort on their end.

Updates:

• the simplest solution is to use the paypal donate SDK to add a donate button to the website. When clicked the user can authenticate and submit the donation via paypal in a popup over the BPR site. https://developer.paypal.com/sdk/donate/
• Investigate if there is a response from Paypal and if this can be used to save amounts to the db to track donations

[kasugaijin]

Issue covered by https://github.com/kasugaijin/baja-pet-rescue/issues/133