This has the benefit of not adding `.preview` divs everywhere, anytime we use `parse()`, and also to un-tie the position of the preview blocks from the result of the helper. This means that templates that call `parse` and have some extra markup after that are not constrained anymore.
This is effectively an alternative, better way to fix https://github.com/thelounge/lounge/issues/1343, but the initial fix that was put in place (https://github.com/thelounge/lounge/pull/1347) is still relevant, for example to make sure a preview stays hidden (and does not add extra margin/padding/etc.) if the link does not prefetch.
- 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
