Move the ImageJ update bootstrap.js script from Fiji

Fiji used to be the home of the updater, but that has been long ago, and
the updater learnt really a lot of tricks in the meantime (including being
robust, thanks to a ton of regression tests that are executed faithfully
by ImageJ's Jenkins server).

Therefore, the bootstrap.js script, intended to be run to initialize the
updater (or fix it), should also move...

To its new home: https://github.com/imagej/imagej-updater

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Move Fiji's script to keep bootstrap.js' URLs up-to-date

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Teach bootstrap.js to output up-to-date .jar URLs

This merges the code from the Beanshell that just moved from Fiji into
bootstrap.js proper.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Update bootstrap.js' .jar URLs

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
In command-line mode, reuse local .jar files

When the ImageJ update bootstrapper already downloaded the appropriate
.jar files, we should just go ahead and use them. Of course, that means
that we should never, ever upload -SNAPSHOT versions to the update site,
but we should not do that regardless, anyway...

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