- Load up to 5 previews per message (to avoid abuse)
- Do not load multiple times the same URL
- Prepare preview containers per message instead of appending (to maintain correct order)
- Store an array of previews instead of a single preview in `Msg` objects
- Consolidate preview rendering for new messages and upon refresh/load history (when rendering entire channels)
- Update `parse` tests to reflect previous point
- Add test for multiple URLs
- Switch preview tests from `assert` API to `expect` API
