cargo-cov HTML template improvements

[kennytm]

• Change sortable.js to something smaller.
• Allow hiding skipped lines
• Support .scrollIntoViewIfNeeded() outside of Chrome
• Test on IE

[sesam]

Probably scratch "Test on IE" and possibly also "support .scrollIntoViewIfNeede" since also Microsoft is switching to the Chromium engine.
And maybe split this into multiple issues -or make it a checklist :)

• Replace or squash sortable.js to decrease file size
• Allow hiding skipped lines