Make retaining active storage values optional

Problem/Motivation

We currently merge into the active storage, respecting existing values.

While this is a common use case, there is also a use case for overwriting the active storage to reflect what would be a "clean" initial install state.

Proposed resolution

Introduce an $overwrite argument to ConfigSyncInitializer::initialize() to trigger overwriting of changes in the active storage.

Remaining tasks

User interface changes

A new checkbox at the initialization screen allows selecting whether to retain customizations in the active storage.

API changes

Data model changes