This commit introduces a new `Problem` model, analogous to `Crop`, to allow you to track problems you have on your plantings (e.g., aphids on tomatoes).
Key features:
- A new `Problem` model that can be curated by admins (`problem_wranglers`).
- You can associate problems with your plantings and upload photos of the problems.
- Aggregated problem information is displayed on the crop detail page (e.g., "Problems: aphids (27), blight (13)").
- You can mention problems in posts (e.g., `[aphids](problem)`), which automatically links to the problem's page.
- Admin functionality for reviewing and approving new problem suggestions.
