This change adds the babel preset for typescript and a babel compiler

that matches it. With this, .ts files can be added to JavaScript

bundles and will work correctly.
I've stuck a tsconfig.json file in the root. This isn't critical because

we don't use tsc to compile .ts to .js (that job is handled by babel),

but this allows us to use things like tsserver for the dev environment

without getting errors about non-ES3 constructs.

Used some .ts files in the Review Board codebase.