WIP: 404 in favor of 400 in progress controller

[thomasplevy]

This is a early WIP to per #181

(I named the branch wrong)

@eri-trabiccolo can you have a look through the changes here and see if I'm track per our discussion in #181

I'm submitting this early because I want to make some progress on this but I'm feeling uncertain whether I'm moving in the right direction.

This still requires the following before it can be submitted as a full PR and merged:

• Tests for new server method llms_rest_page_restricted()
• Logic for GET requests on the progress controller per discussion in Student progress should return a 404, not a 400 validation error, when requesting progress for an invalid post id #181
• A review of the existing DELETE requests -- I believe they're solid but they may need updating
• A review of the generated links. If a 3rd party adds a completeable post type and uses this endpoint to record completion the resulting links will (potentially) lead to 404s
• Tests to simulate progress recording by a 3rd party for a custom post type (we can filter to allow completion of a page and see what happens. It should work).

[eri-trabiccolo]

I was following this approach on my branch https://github.com/eri-trabiccolo/lifterlms-rest/tree/restrictions
eri-trabiccolo@debe8b5#diff-5ce7aeb33633a2220e61908290ff5aedR806-R815

I needed to set the query's queried_object and the queried_object_id as well, I cannot remember exactly why at the moment. But just in case you'd find some inconsistencies during your tests, take this into account.

[thomasplevy]

I thought I saw you start doing something like this somewhere but I couldn't find it!

[thomasplevy]

And the reason I added this as a function was because I was pretty sure you were doing something like this and figured we'd need it elsewhere at somepoint

[eri-trabiccolo]

I think you did it right.
I did it inside the "loop" in the post controller because I needed it in the posts loop but I will adapt my code using the new function. Looks more clean.

[thomasplevy]

@eri-trabiccolo I'm working on reworking llms_page_restricted() to be useable outside a loop. This function is some very stringy code and needs a refactor. I'm taking it on instead of writing a hack around it.