Move package to new npm namespace

Now that we've reclaimed the @gitlab namespace in npm, let's rename our packages.

1. Update the namespace from @gitlab-org to @gitlab.
2. Update the package from gitlab-ui to just ui, since the namespace makes this redundant.

Ultimately we can change our package imports in gitlab-ce like so:

// old
import { ProgressBar } from '@gitlab-org/gitlab-ui';

// new
import { ProgressBar } from '@gitlab/ui';

Let's also make sure we deprecate the old package namespace with npm deprecate. This will place a friendly warning at the top of the npm page, and in the console when a user installs the old package (we need to make sure this warning message doesn't break old GitLab stable branches).

See jade (renamed to pug) as an example of this.