Warning: Accessing PropTypes via the main React package is deprecated, and will be removed in React v16.0

We should start accessing PropTypes from the prop-types package, as requested by React warnings such as:

Warning: Accessing PropTypes via the main React package is deprecated, and will be removed in React v16.0. Use the latest available v15.* prop-types package from npm instead. For info on usage, compatibility, migration and more, see https://fb.me/prop-types-docs

We will need to do this for all the UI modules and stripes-core, but it would be too tedious to file separate housekeeping tasks for each of those.