Expose version history of portals

[laurenwalker]

This idea was brought up during the ADC oct 2020 training.

We should add a tool to view the previous versions of portals, with the ability to click on a previous version and revert back to that version. Google docs and Wordpress do something similar. We would probably have to build a "preview" portal view that renders a portal without needing to fetch it first (should be straightforward). That preview view could be released first as a "preview portal/draft" feature while editing

When this feature is developed, we should build it so that it can be reused by the EML editor as well