Support for before/after pages.

[Antoliny0919]

Currently, some templates do not support before/after.
For pages without support, when the user modifies a certain part, both the before and after states are updated identically.

In other words, comparison becomes impossible.

At present, templates related to tabular and stacked inlines do not support before/after, and there are also suspicions that custom widgets may have the same issue.

This needs to be properly resolved (at least for all templates within the admin folder) before it can be practically usable.