Fix_Updater.js: In command-line mode, reuse local .jar files

When the ImageJ update bootstrapper already

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Move the ImageJ update bootstrap.js script to imagej-updater

As per commit f131630dcbde5f7b3c64fe5bb1c993b0e97fa546 in
https://github.com/imagej/imagej-updater, the Javascript intended
to bootstrap or fix the ImageJ updater lives in the imagej-updater
project instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Move the script to update bootstrap.js' URLs into imagej-updater

As per 964b40ddd7d1791547f4ab5112616486c21a6601 in
https://github.com/imagej/imagej-updater, the script to update the URLs
in the ImageJ bootstrapper script lives in imagej-updater. A later
commit even merges that code into the bootstrap.js script itself.

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