[Feature Request]: Remove JSON files & fix activity thumbnails

[BenHenning]

Is your feature request related to a problem? Please describe.

With #5652 and the removal of Gradle, we no longer need to support loading JSON lesson files.

Describe the solution you'd like

Removal of all JSOn lesson files and corresponding JSON loading code, along with removal of any gating code/logic across the codebase (i.e. LoadLessonProtosFromAssets can be completely removed).

Note that while the clean-up is straightforward, we can't actually move forward with this change without also fixing the fact that thumbnails do not load correctly with proto files. This needs to be fixed properly as well (ideally by introducing some sort of dev-specific override for loading certain thumbnails so that we can force them to be local, dev-only versions).

Describe alternatives you've considered

There's not a strong alternative for the JSON clean-up as proto is the predominant format the app will be using for lessons moving forward. However, the exact approach for fixing the thumbnail issue is unclear, and there are likely many alternative solutions possible.

Additional context

No response