U8N WXD

Per @seanlip this PR doesn't need to go through backup server testing because we can't test mailchimp on the backup server

I've replied to or resolved my responded-to comments

Here's my analysis of this issue for future reference: https://gist.github.com/U8NWXD/628433fd22d5def7fe07f61e31c25169

@chris7716 @StephenYu2018 I have a prototype solution working that's documented in https://gist.github.com/U8NWXD/628433fd22d5def7fe07f61e31c25169#implemented-solution. Here's a summary: When an exploration is reverted from e.g. version 5 to version 2, creating version 6,...

> what do you mean by "a field of the exploration"? I mean any translatable text in the exploration. For example, the text on a card or the text "Continue"...

> I think the team members should have some sort of way to observe what changed between the version before the revert and the version after the revert. Additionally, I...

I chatted with @seanlip, and his opinion was that we implement the simple behavior for now because reverts are pretty unusual, and it's more faithful to the conventional idea of...

Assigning @divyanshsharma2280 since they were the first to follow our [instructions for taking up a good first issue](https://github.com/oppia/oppia/wiki/Contributing-code-to-Oppia#choosing-a-good-first-issue).