Handle case where user lacking some setting caps loads a snapshot into the Customizer

[xwp]

Handle case where user lacking some setting caps loads a snapshot into the Customizer

[westonruter]

This should be addressed by changesets. A changeset UUID is always supplied when accessing the preview and all pre-saved settings in the changeset should be made available in the preview, even though the user cannot modify them (write them into the changeset).

[jeffpaul]

@westonruter So this should be resolved via #102?

[westonruter]

@jeffpaul I believe so. Just needs testing to confirm.

[PatelUtkarsh]

@westonruter Tested and it works as expected. The user can load changeset and make changes to the section which is accessible to it, other section data preserve on save of that.

Created snapshot with a user A who doesn't have access to some section and added some data, Added new data in changeset with User B(admin) of which user A doesn't have access to, Opened changeset with user A and edited/updated new data, It preserves user B's changes in changeset.
CC: @jeffpaul.