Before commit 28caabecd, it was clear which budgets were in draft mode
because their phase was "drafting".
Now the phase isn't "drafting" anymore, so we have to make it clear
somehow that the budget is a draft.
I'm using styles similar to the ones we added in commit 2f636eaf7 for
completed budgets but at the same time making them slightly different so
it's easy to differenciate completed and drafting budgets.
We're going to change it a bit, and using a component allows us to use
the `header` method and easily add a title to this page, which lacked
the `<title>` tag.
