DRY up SnapshotsController with a before_filter
DRY up SnapshotsController with a before_filter

In a few actions we had repeated the same finding of the Snapshot
instance from the id param and setting it as an instance variable in
this controller. To avoid repetition, I'm moving this to a
before_filter. This will make the code more maintainable.

Change-Id: I795192162fc8966f3bd0709620962ed8427fc315

diffux/diffux,kalw/diffux,diffux/diffux,kalw/diffux,kalw/diffux,diffux/diffux