qt: Add privacy feature to Overview page

Summary:
This is a backport of core#16432 [2/2]

https://github.com/bitcoin/bitcoin/pull/16432/commits/8d75115844baefe5cad4d82ec8dce001dd16eb9c

Depends on D9919

Test Plan:
ninja && src/qt/bitcoin-qt

Toggle the privacy option with Shift + Ctrl + M or through the menu.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9920