Rename SectionHiddenStage model

[uponthesun]

Testing story

• Drone test pass
• Looking for recommendations on what to manually test, if anything

Reviewer Checklist:

• Tests provide adequate coverage
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[dmcavoy]

Looks good Winter!

For manual testing I would try:

1. On a teacher account make a section that has some students in it
2. Go to a script and hide a lesson
3. Go into one of the students accounts and check that viewing that script you don't see that lesson

Let me know if you need more details

[davidsbailey]

Great idea Dani -- I only see an eyes test (no regular ui test) for this feature: https://github.com/code-dot-org/code-dot-org/blob/4008770ce7c309df7ddb39ff5064e104d1222545/dashboard/test/ui/features/learning_platform/hidden_stages_eyes.feature#L4-L3 and the eyes test doesn't really do any validation besides the visual validation. I'm always worried someone will incorrectly accept a new baseline when that's our only end-to-end coverage.