qt: make BitcoinGui::config a const ref

Summary: The callsites all assume that config is never a nullptr, so enforce it by using a const reference rather than a bare pointer.

Test Plan:
ninja all check-all

run bitcoin-qt

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18145