Single boot script for embedded chat

Closes #2219 (closed)

The changes related to vue left menu included decision whether the chat-layout is rendered with the left menu in one page, or in an iframe and including boot script based on that decision.

This caused the /~embed route (e.g. https://gitter.im/gitterHQ/sidecar-demo/~embed) including the boot script twice and hence submitting two messages.

Testing strategy

1. enable vue-left-menu toggle
2. go to any room with /~embed route and submit a message - only one message should be displayed in message feed
3. normal chat (without /~embed) still submits messages