Remove Sandstorm support (#13773)

.circleci/config.yml

.meteor/packages

.meteor/versions

.sandstorm/.gitignore

.sandstorm/CHANGELOG.md

.sandstorm/README.md

.sandstorm/Vagrantfile

.sandstorm/build.sh

.sandstorm/description.md

.sandstorm/global-setup.sh

.sandstorm/launcher.sh

.sandstorm/pgp-keyring

.sandstorm/pgp-signature

.sandstorm/rocket.chat-128.svg

.sandstorm/rocket.chat-150.svg

.sandstorm/rocket.chat-24.svg

.sandstorm/sandstorm-pkgdef.capnp

.sandstorm/screenshot1.png

.sandstorm/screenshot2.png

.sandstorm/screenshot3.png