Allow for pinning scenarios/datasets in model-check report

[chrispcampbell]

Currently in the model-check UI, scenario and dataset items are always sorted according to the number of differences reported for each item (items with the most differences bubble up to the top of the list). This is OK in the summary view, but in the detail view, this behavior can make it difficult to review changes for a particular scenario or dataset because the view always scrolls to the top when navigating between items.

One solution to this would be to introduce a "pinning" feature in the model-check report. If you pin an item, it goes to a section at the top of the summary/detail page with a fixed ordering, so when you navigate between pages, the same items appear in a consistent location.

Nice-to-haves:

• Allow the user to reorder the pinned items via drag and drop
• Save the set of pinned items in LocalStorage so that they persist between builds

[chrispcampbell]

Re: the nice-to-haves above, I did implement drag and drop in the summary view, but decided not to include the pinned items in the summary view after all; we can consider restoring that code later. I didn't do the LocalStorage thing for now because I will have to figure out a better way to have stable scenario identifiers between runs, so I will work on that in a later issue.

With these changes in place, in the detail view, there is now a context menu for each comparison graph box that the user can access by right-clicking on the box title or the row title:

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Selecting the "Pin [Item]" option will make that item go to the top of the detail view for all pages in that category:

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I didn't want to get too bogged down with implementing reordering the pinned items via drag and drop, so for now, when there are multiple pinned items, you can select "Move [Item] to Top" to have some basic reordering support:

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You can also pin an entire row of scenarios at once in the "Comparisons by output" detail view:

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As part of this I improved the section header style in the detail views to include a horizontal line to help visually separate the sections.