settings: Add resetSettings() method

Summary:
Allows the GUI to clear settings.json file and save settings.json.bak file when
GUI "Reset Options" button is pressed or -resetguisettings command line option
is used. (GUI code already backs up and resets the "guisettings.ini" file this
way, so this just makes the same behavior possible for "settings.json")

This concludes backport of core#15936
https://github.com/bitcoin/bitcoin/pull/15936/commits/f9fdcec7e932843a91ddf7f377e00bd2a6efb82a

Depends on D15066

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15067