Add "Send via Paypal" button to admin/cash_outs/pending

If cash out method is Paypal, add a button to that specific cash out request that sends the payment via Paypal API. This probably should be batched as Paypal's Synchronous mode will be deprecated.

- Payment must sent as USD (no conversions)
- Successful transaction should trigger the "Mark as Sent" action in the Admin
- (bonus) Add transaction ID/details to the "Mark as Sent" email

Start at the admin/cash_outs/pending view and trace the existing cash-out and “Mark as Sent” actions. Read how Paypal cash-outs are represented and how the Paypal API is currently integrated, then determine how batching and USD-only payments fit the issue requirements. Done means a Paypal payment can be sent, successful transactions trigger “Mark as Sent,” and the optional email details are addressed.

Written by the indexing model from the issue text.