HTML Coverage report improvements

[Xenomega]

This PR adds buttons to "expand all", "collapse all", "expand empty", and "collapse empty" contracts in our coverage reports.

It also simplifies some of the surrounding code.

Expand/collapse all is OK, but it seems like we need better navigatability. Outstanding tasks are:

• Add a file explorer on the left (similar to GitHub's split pane view for changed files. Needs support to click + jump to a file (should auto expand).
• Add some kind of headings so when you click a file in the explorer, it adds the heading to the page URL (so you can reference it later).
• Add auto-expand to the panels when a selection is made within them. Specifically, if you search for a word like "function" with all source files collapsed, it will show you results but not bring you to them, because the collapsible containers do not expand.

[anishnaik]

bing bong