First attempt at adding colour to articles

Added simple boolean colour state to every article in an event.

Thoughts,

- It's far too tedious to set the colour of everything, so _light_ should be pretty agnostic, or default should be no colour. But then default things have a tendency to be ignored.

-  I'm not sure how many _tones_ we want. I've gone for a light blue to represent entertaining content and dark blue to represent serious stuff.

- I'm wondering if the existing tone or tags would be able to set a initial state.

- Having played around with it, colour makes a lot of sense to me.

@stephanfowler @mattandrews