Revisit column defintions - Add NULL constraints and indexes

[jomey]

Most of our column definitions don't have the null constraint.
For example - an instrument can be currently be added without a name.
This can be improved by adding

Column(String(), nullable=False)

Second review while doing a swoop through the tables is to check for needed indexes.
From the above example, it would also be good to have an index on names since that is a very frequent query we will have

Column(String(), nullable=False, index=True)