MirrorWebsite: make job scheduling less fragile

Up to now, we had an elaborate, clever, and hence pretty fragile way
to schedule mirror jobs. As a consequence, things broke down when we
tried it on file:// URLs.

Instead of pretending that we're clever, let's use a simple (and
therefore robust) way to schedule jobs.

And sure enough, the executor service gets shut down properly now.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
MirrorWebsite: print progress

It is only sort of a progress because the total keeps changing pretty
often, but it is better than nothing.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
MirrorWebsite: special-case ij.tgz-style mirroring

We have to download a whopping 1.7G file every night now and unpack
it into a local directory. Still, we would like to rewire the absolute
URLs in <a> tags to relative ones whenever possible.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
MirrorWebsite: report all broken links at end

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>