HTML support for #7

Adds an input element for the commit-ish. On wide screens, the userRepo
input and the commitIsh input are stacked horizontally; on smaller
screens, they stack vertically.
linkStackTrace() signature support for #7

Not implemented, but linkStackTrace() now takes a commitIsh parameter
(null to indicate “keep GitHub’s default”), and onSubmit() supplies it.
Fully implement #7: replace commit-ishs
Fix newlines before/after “generated by…” footer

I used to concat lines with ‘\n’; when I changed it to Array.join(‘\n’),
I didn’t update the newline handling for the footer, which meant that
the footer was now on the same line as the last stack trace line.