Migrate platform plugins to external repository

See:
https://github.com/scijava/scijava-plugins/platforms
Migrate uploader plugins to external repositories

See:
* https://github.com/imagej/imagej-plugins-uploaders-ssh
* https://github.com/imagej/imagej-plugins-uploaders-webdav
Migrate text format plugins to external repositories

See:
* https://github.com/scijava/scijava-plugins-text-markdown
* https://github.com/scijava/scijava-plugins-text-plain
Merge branch 'split-plugins'

This branch splits several more plugins into external repositories.

Platform handlers:
* https://github.com/scijava/scijava-plugins/platforms

Uploader protocols:
* https://github.com/imagej/imagej-plugins-uploaders-ssh
* https://github.com/imagej/imagej-plugins-uploaders-webdav

Text formats:
* https://github.com/scijava/scijava-plugins-text-markdown
* https://github.com/scijava/scijava-plugins-text-plain