Add current user to SuperBlock

[zemogle]

Log the observer's username in SuperBlock. This info is available in the FITS headers, so we could potentially backfill past SuperBlocks.

[zemogle]

Instead of making this a field on the SuperBlock model, I've made it an extra model, a little like a many to many model. There are 2 advantages:

1. We don't have to create a new column which will be blank for all the past Superblocks
2. We can have multiple observers, if for example we manually combined blocks submitted by different people.

I also added an observers property on SuperBlock which will give a list of observers (not a queryset).

[talister]

Deployed