Allow paying with Paygreen via redirect URL.

[alexsegura]

Fixes #1941

[r0xsh]

The feature is working as expected and should work on modern devices.
I was worried about using external browser on which we don't have any control and know which browser the user is using, i tried it with Firefox focus and still the payment work fine 👍
However on lower spec devices i was worried about the OOM background app killer feature present on both iOS and Android (it's not always about memory available, it's can be killed for battery management), so i tried killing CoopCycle while i was in the browser to see if the state restoration was well handled, sadly it is not and lead to an infinite loading state :/

output.mp4