unhide script and warn when assigning it from the script overview page

[davidsbailey]

Background

Spec line item: https://docs.google.com/document/d/17X2bEVK4YO4gymmVEzAnDYCYTjE5xHQTlDSAZbEprTo/edit#bookmark=id.hko4b4h5at6x

Terminology

The term "hidden script" is ambiguous. This PR talks about hiding scripts for certain sections as determined by the SectionHiddenScript model, which a teacher controls by that script's course's CourseOverview page. No part of this PR has anything to do with the hidden property on the Script model, which is controlled via the levelbuilder UI.

Description

1. if a script is hidden for a section, and the section is assigned to that script via SectionsController.update, unhide the script for that section.
2. if the script is assigned via the AssignToSection button on the ScriptOverview page, show a more accurate warning in the dialog prior to unhiding and assigning.

Future work

3. show a more accurate warning when the script is assigned via EditSectionForm, pending in warn before unhiding and assigning script via EditSectionForm #24682

Screenshots

[islemaster]

Looks great! Well-tested.

[islemaster]

It looks like the last bit of this type signature is wrong; just {Hash<string,number[]>}?

[davidsbailey]

Good catch! copy-pasta.