Allow a snapshot to be split up into multiple snapshots

[westonruter]

If a subset of changes from a snapshot need to be isolated into a separate snapshot, such as scheduling out a subset of settings ahead of other settings being applied, we should be able to split up a snapshot into multiple snapshots.

Given the UI for listing out the available snapshots (#12), a user could open a snapshot in an edit post screen, and from there the list of settings in the snapshot could be listed. Along with each setting there could be a checkbox to allow it to be selected, with a button then for taking the selected settings and moving them into a separate snapshot.