Display volunteer state on the volunteer list page

[beverloo]

When looking at the list of volunteers, we currently list all of them with their avatar, name and role. However, we should distinguish between a few states here:

• Available (w/ time until next shift?)
• Unavailable (w/ time until available or next shift?)
• Active with shift title and location (w/ time until shift done?)

We'll have to watch out to not make #21 worse.

[beverloo]

Fixed by:
cf152f5
9062abb