Replace PropTypes with external lib

[unclecheese]

In React 16, the PropTypes library has been removed and put into its own prop-types external library.

Pull Requests

• All proptypes replaced with external lib silverstripe-campaign-admin#118
• All proptypes replaced with external lib silverstripe-cms#2264
• All proptypes replaced with external lib silverstripe-asset-admin#836
• Remove references to React.PropTypes #645
• Remove references to React.PropTypes silverstripe-versioned-admin#63
• Update docs to reflect prop-types change silverstripe-framework#8385
• Add proptypes external webpack-config#27

[robbieaverill]

Seems like there's still open PRs for this

[lukereative]

I think it was auto-closed because one of the associated issues was merged – I didn't close it manually.