Updater: Add a DropBox URL mapper

DropBox stopped having convenient URLs, now we have to work around stuff.

Reported by Jan Brocher.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater: hide Dropbox URL handling behind Util methods

Who knows what other handling we require in the future... ;-)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Updater: always go through Util to access URLs

That way, we automatically get Dropbox URL handling.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Ignore Dropbox' inability to serve HEAD requests

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'dropbox'

This works around the Dropbox issues reported by Jan Brocher.

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