Missing metadata causes JS errors

[bxm156]

Relates to #34.

When certain metadata fields are not provided, their use in the frontend JS causes errors.

For example, if metadata is missing, the following JS error appears in the console.

Uncaught (in promise) TypeError: Cannot read property 'company_profile_url' of undefined
    at User.render (User.jsx:28)
    at ReactCompositeComponent.js?d2b3:796
    at measureLifeCyclePerf (ReactCompositeComponent.js?d2b3:75)
    at ReactCompositeComponentWrapper._renderValidatedComponentWithoutOwnerOrContext (ReactCompositeComponent.js?d2b3:795)
    at ReactCompositeComponentWrapper._renderValidatedComponent (ReactCompositeComponent.js?d2b3:822)
    at ReactCompositeComponentWrapper._updateRenderedComponent (ReactCompositeComponent.js?d2b3:746)
    at ReactCompositeComponentWrapper._performComponentUpdate (ReactCompositeComponent.js?d2b3:724)
    at ReactCompositeComponentWrapper.updateComponent (ReactCompositeComponent.js?d2b3:645)
    at ReactCompositeComponentWrapper.receiveComponent (ReactCompositeComponent.js?d2b3:547)
    at Object.receiveComponent (ReactReconciler.js?399b:125)