make the location of avatar images configurable via aula.yaml

[fisx]

this is needed for the test suite to have its own location for avatar images.

we already use a temp file in tests here: https://github.com/liqd/aula/blob/31d32df18871a465cbc9a16d50df635555484eb4/tests/Frontend/Page/AdminSpec.hs#L69

https://hackage.haskell.org/package/temporary-1.2.0.4/docs/System-IO-Temp.html also has something for creating directories.

i think the biggest challenge with this will be that we need the config in lots of places deep in some pure computations where we have to pass it down by hand.

while we're at it, we might as well look what bits and pieces of persistent state we are not mentioning in aula.yaml yet, and add them. here is another list, from a whiteboard session this morning. is it complete?:

• acid-state
• avatar images
• /static
• event log
• system log (?)
• aula.yaml

[andorp]

I think the list is complete.

[andorp]

Fixed in #931