Enable issue tracking by default, and add an alt-I shortcut.

More often than not, I believe people will want issue tracking enabled, but
it's annoying to click the checkbox every time.

This change defaults issue tracking to on by default, which appears to have
been the original intent in the HTML.

Users can turn issue tracking off quickly by pressing Alt-I in the textbox.

Documentation has been updated to reflect these changes.

Tested Alt-I. It works.

Tested that by default, issues are opened.