(FR #508) New Paypal adapter

* Validates before redirect
* Stage "src" and "srt" in a way that makes it possible to recur
forever (the default).
* revert "_xclick" to "_donations": it prints better text in the
hosted pages

Deployment -- must configure these variables:
* $wgPaypalGatewayAccountInfo['default']['AccountEmail']
* $wgPaypalGatewayReturnURL

Gateway invocation requires these parameters:
* redirect=1
* gateway=paypal
* currency_code
* amount
* optionally, recurring=1

Change-Id: I81d234947d6256f360132bd627ef8b3c39d2f89b
Merge "(FR #508) New Paypal adapter"