use assign_script

[Bjvanminnen]

Realized in a discussion with Erin Bond today that when assigning a script to a section via the script overview page, assigned_at was not getting set.

This PR makes it so that we use student.assign_script instead of User.track_script_progress. Both potentially create a new UserScript, but this one sets assigned_at instead of started_at/last_progress_at. This is the expected behavior, and matches what happens when you assign a script via teacher-dashboard as well.

[islemaster]

Looks like User.track_script_progress is not documented. What is the semantic difference between these two methods - when should we use one or the other? Should one of these be removed? Should UserScript.first_or_create be modified to always set assigned_at if a new record is created, or should this be baked into our schema?

[Bjvanminnen]

My understanding is that track_script_progress is just meant to say "I did a level in this script" whereas assign_script is meant to say "I'm in a section and that section has been assigned to this script". I think therefore it's by design that only one sets assigned_at. I just hadn't done due diligence when initially writing this code and chose the wrong one (at the time I was just interested in seeing that a UserScript got created, not realizing there were different ways to do this).

[islemaster]

Sounds good. I'd recommend adding related comments to the two methods just to make this more obvious in the future.