Fix the way instruments are stored for events
Created August 16, 2017. Copied from redmine (https://bugs.ligo.org/redmine/issues/5694)
Instruments are currently associated with events by a string like "H1,L1" or "H1,L1,V1". This is an ineffective way of doing it and prevents efficient instrument-based queries.
We should create an instruments model and just have a many-to-many relationship with events (may need to create a go-between like "labelling").
I think there is also an 'ifos' variable: we should resolve the redundancy issue if that's the case.