Export to HTML - make page less overwhelming

[tidoust]

Via internal feedback.

The HTML page that gets generated contains:

• A schedule table. For a TPAC event, table contains about ~200 cells.
• A view per group that lists meetings, joint meetings, and possible conflicts. >70 groups for a typical TPAC event.

That is a lot of information. The page is more a temporary dump of data meant for auditing purpose before we push the data to the calendar than a functional page aimed at distribution. But then we do distribute the page among chairs, e.g. see TPAC 2025 draft schedule page.

Without spending too much time on a transient page, it should be possible to improve layout to make the page more directly readable. For example:

1. Merging cells in the table where possible. We do that already in the spreadsheet and in the final schedule page.
2. Merging conflict warnings where possible, either when a given slot creates a conflict with multiple groups, or when multiple contiguous slots create a conflict with one or more groups. That logic could be useful in the spreadsheet as well.
3. Turning the group view into expandable sections.
4. Adding a bit of JavaScript to give users the ability to only show a particular group (or a short list of them?)

An alternative would be to inject the data into the calendar right away and prepare the draft schedule directly in the final schedule page, but that sort of dismisses the point of reviewing the data before it becomes calendar entries (once created, calendar entries can be "canceled" but manual intervention by systeam is needed to "delete" them. The fewer changes we need to make to calendar entries, the better).