Prevent Editing Hosted Events on BE

[vkoves]

Description

This PR not only prevents users from editing hosted events on the BE, but adds a ton of schedules_controller tests that act as sanity checks and specifically check for cases around hosted events.

Since Matt's FE editing lock PR has not been merged, this means you should have an error saving if you modify anything on a hosted event other than the category, which is the intended behavior!

Part of #370.

Type of Pull Request

Based on the contributor's guide, this PR is of type:

• [x] Development (feature-branch -> dev)
• [ ] Hotfix (hotfix-branch -> master)
• [ ] Release (release-branch -> master)
• [ ] Other

Requestor Checklist

Requestor: Put an x in all that apply. You can check boxes after the PR has been made.

Reviewer: If you see an item that is not checked that you believe should be, comment on that as part of your review.

• [x] Code Quality: I have written tests to ensure that my changes work and handle edge cases
• [ ] Code Quality: I have documented my changes thoroughly (using JSDoc in Javascript)
• [x] Process: I have linked relevant issues, marking issues that this PR resolves
• [x] Process: I have requested at least as many reviews required for this PR type (2 or 3)
• [x] Process: I have added this PR to the relevant quarterly milestone
• [ ] Process: I have tested this PR locally and verified it does what it should

How This Has Been Tested

Tested manually by making sure I could still use my schedule and by making sure I was prevented from making any changes to hosted events. Also ran automated tests.

[vkoves]

Update: I discovered myself what the bug was and fixed it since I opened this PR. This should now be working properly!

[vkoves]

@Watercycle - updated!