Provide Projector sources as subtree

[vzhukovs]

This changes proposal provides several improvements in build process by providing Projector server and client as dedicated git subtree in the following repository. This subtree will rebase against master branches automatically through the rebase-projector-sources.yml GitHub action. Automatical rebase is performed by cron every 4 hour.

Also each asset that need to be provided to main container build is become independent from the host os as build of each asset is performed in dedicate docker container. So there is no need to have installed java on the host.

As far as devfile-plugin now has the functionality that differs from devfile support it is renamed to che-plugin. This allows us instead of patching the Projector's sources, provide the necessary fixes through the following plugin.

To manual rebase the source code from the host by calling ./projector.sh rebase it is necessary to provide the additional remotes by executing the following git commands from the repository root:

$ git remote add upstream-projector-server https://github.com/JetBrains/projector-server
$ git fetch upstream-projector-server master
$ git remote add upstream-projector-client https://github.com/JetBrains/projector-client
$ git fetch upstream-projector-client master

Patches directory removed as it doesn't need anymore.

Needed for https://issues.redhat.com/browse/CRW-2736

Reviewable changes made for the upstream provided in the following commit: 7c99945