Add `moment` as a development dependency

What does this MR do?

This will add moment@2.22.2 in devDependencies which is required by pikaday added in gitlab-org/gitlab-ui!616 (merged). Package moment is already included as a development dependency in gitlab-ui, see gitlab-org/gitlab-ui@0bbd3f88.

See also relevant slack discussion (internal).

Benefits

This will avoid running into the following warning when running yarn start locally:

WARN  Compiled with 1 warnings
WARN  in ./node_modules/pikaday/pikaday.js
Module not found: Error: Can't resolve 'moment' in '/path/to/design.gitlab.com/node_modules/pikaday'