[QT] Restrict third party tx URL to https scheme only

Summary:
Improve the user security by enforcing https URLs.

Test Plan:

./src/qt/bitcoin-qt

Go to Settings=>Options=>Display and try to validate an URL with a bad
scheme.

Reviewers: #bitcoin_abc, josephroyking

Reviewed By: #bitcoin_abc, josephroyking

Subscribers: josephroyking

Differential Revision: https://reviews.bitcoinabc.org/D9292