GUI: If BIP70 is disabled, attempt to fall back to BIP21 parsing

Summary:
Commit 1:

GUI: If BIP70 is disabled, attempt to fall back to BIP21 parsing

GUI: If BIP70 is disabled, attempt to fall back to BIP21 parsing

Commit 2:

GUI: If BIP70 is disabled, give a proper error when trying to open a payment request file

https://github.com/bitcoin/bitcoin/pull/15063/commits/113f0004bec2dd203502f2a9bcba93676d5b9cda

This is a backport of core#15063

Test Plan:

cmake .. -GNinja -DENABLE_BIP70=OFF
ninja all check-all`

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9897