Use only absolute paths in client-side scripts

In some places absolute paths were already used, but far from everywhere.
Using relative paths creates an unfortunate dependency on the path of the
page, which is often varying (review front-page could be either /r/NNN or
/showreview?id=NNN, which makes a huge difference for relative paths.)

This hasn't been a problem because (almost) all pages are delivered with a

  <base href="/">

element, but that creates its own problems and is quite ugly.
Use absolute paths in page header links
Use absolute paths to static resources
Use absolute paths on /showcommit page; drop <base href="/"> element

Having the <base href="/"> element causes the path name links in the list
of modified files to be to /#fNNN instead of to the named anchors on the
page.  This also means that clicking such a link before the script that
sets event handlers on the links has run navigates to / by mistake.
Register event handlers on path name links sooner

Instead of waiting until the whole page has been parsed, register the
links as soon as the table of changed files has been parsed.  That way
there's little risk that the user manages to click any of the links
before the event handlers have been registered.