tidy up the `create=y` flow for non-logged in users

[tackley]

When a room url is access with the create=y parameter (as generated by the internally used /talk slack extension), if no premium credentials are available, the following screen is displayed:

This is to avoid accidentally creating a non-premium limited-access room, which can cause subsequent confusion.

However, it's not entirely clear why this is being shown, and the button to proceed anyway and create a "free" room doesn't work.

• Improve the messaging to be clearer that you're seeing this message because you are not logged in, and we don't want to automatically create a limited access room, so we're waiting for someone else to do so. Perhaps describe the workaround of logging in and then clicking the button in slack again.
• Make the button more clearly labelled to indicate "I'm fine with a limited access room, just do it". And make it actually work.

See slack discussion for more info.

[bbondy]

I think this issue can be simplified to just:
Make the button that says Start free call (up to 4 people) work.