Allow a snapshot to be reverted

[westonruter]

Before a snapshot is published (before its settings are saved), we should store the non-previewed values for the settings that are being overridden. These can be stored in the snapshot's meta. With this data available, we would be able to

Allow snapshots to store the previous state overridden for each setting saved. When viewing a snapshot that has been published, this pre-applied setting data could be shown in the diff view (see #18) instead of the current live settings.

When a snapshot has reversion data available, a button should be available from the edit post screen (#18) to revert the snapshot.

[westonruter]

See also #21666: Customizer reset/undo/revert