Releasing to the plugin directory

[nylen]

After #1025 we should be ready to release the zip file resulting from npm run package-plugin to the plugin directory.

Not covered in that PR: we should make sure that each released version of the plugin has a unique version number in gutenberg.php, and that a git tag is present for each version. For example:

git fetch origin
git checkout origin/master -B master
npm run package-plugin
# upload the plugin to the directory
git tag v0.1.0
git push --tags

Worth considering to automate this tagging and some related checks (create this tag automatically; if there is already a tag for the current version, indicate that the version should be updated).

Is there anything else we need to make sure we do for each released version?