Make warning messages more accessible

[joemull]

Fixes #354.
Fixes #269.

Since this issue was written, we now handle most of the login flow in the back office templates, so messages from Django will only appear in two cases: successful login completed, and successful logout completed.

Successful login originates on a different website (the back office), so we don't have the ability to connect the message's placement on the page with the previous context. But the messages are now placed at the top of the body and with the right role attributes to be announced to AT.

Successful logout originates in the account dropdown of the same website. With the messages at the top of the body, the user will encounter them visually on the same area of the page as their preceding action.

The timeout causing the element to disappear after 5 seconds has been removed.

The Edit on GitHub link has been hidden, since we are no longer actively editing the website.

[StephDriver]

I'm not sure about this.

I have tried this with Voice Over, and I can't work out how a user would realise the message was there. On log in / out, I am taken to the start of the page. I have no awareness there is a message for me.

The message itself seems fine, but it's not announced, has no heading, and is not on a landmark. There needs to be some way to know it is there when landing on the page with a screenreader. I wonder whether messages should appear first in the tab order of the page, such that they are read on landing?