Allow users to mark files as "viewed" similar to GitHub PRs

[mikekistler]

It would be nice to allow users to mark files in an ApiView as "viewed" (by that user). GitHub has this feature for PRs and it can be very helpful.

This was suggested by @tjprescott in a discussion we had yesterday. It seems like a nice feature so I wanted to capture it.

### Tasks
- [x] Added switch to mark API Revision as viewed
- [ ] Add UI filter to filter to viewed revisions

[tjprescott]

Generally APIView is only one file, but the analogy would be to be able to check off that you've reviewed an API call/model/etc. When I use APIView for such things, I end up leaving a comment and then scrolling around for un-commented things.

So you could check something off and it appears hidden for example. Almost like how we have a "show diff only" except we could manually hide whole sections on review, whether there's a diff or not.