Prepare for React 16.

* ref(invite): use AtlasKit for invite modal buttons and inputs

- Convert button to AK Button.
- Convert inputs to AK FieldText.

* remove noop, replace with custom empty func

Hides the UI component showing dialout codes and uses the dial input without validating it.

Skips printing error when dial-in numbers is not configured.

- Swap the telephone input with FieldText.
- Swap the dropdown trigger input with FieldText for styling
  consistency with the telephone input.
- Wrap the dropdown trigger so it can be modified with jitsi css.
- Use flexbox to have the trigger and input align horizontally
  but have the input take up width if the trigger is not displayed.

With the upgarde of @atlaskit/icon to 7.0.0, the size prop
essentially became required to maintain its appearance in the
jitsi app, otherwise it'll unexpectedly try to take up the
available space and cause minor display issues.