Improve signin / signup experience

[MartinHowarth]

There are a few problems here, they are closely related so might be solved by one solution (rather than having multiple github issues)

Authentication:

I don't like the user experience of either of the examples here: https://dash.plot.ly/authentication

The reason for that is that users have the signin before getting into using the app. I think that signin/signup should happen as late as possible to streamline the process.

Feedback when a signin/singup request fails

This one is pretty obvious. Currently if these fail then the user is taken to the next page without warning and the UX is weird.

Wait for signin/signup to return before loading the next page

[This is resolved very hackily right now - consider improving alongside a wider fix here]
Slightly tangential to the previous point. Even on successful login, the session['username'] variable is not set before the next page loads, which can impact on its content. We should instead wait for signin to complete before loading the next page.