Preview 404 for branches with uppercase letters

[hypha]

Currently, if the branch name includes uppercase, the preview results in 404.

This might have been the first offender (and a bad practice of mine!): #1000

Artur's input:

Best practice: lowercase is better. But that shouldn't stop us from supporting uppercase too.

We can fix it by doing force lowercase on the deployment code which would be easiest as well and won't require redeploying the static server.

https://github.com/EuroPython/website/blob/ep2025/Makefile#L20 should/could also lowercase the branch name to make it work.

[egeakman]

Closing since we are now using RTD for previews.