Fix iOS asset build - include the embedded chat startup

Attempting to fix the iOS app, found that the main chat widget was empty:

I believe recent refactoring of the webpack config led to the boot script for the mobile page not to be shipped.

This MR includes it again, and at least gets the widget up and running in iOS build:

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Spawned from working on https://gitlab.com/gitlab-org/gitter/gitter-ios-app/merge_requests/8