Ensure crsid field is always lower cased

Part of #14 (closed)

Changes the field for the CRSId primary key on the Account model to use a new field that enforces lower-caseness for the CRSId in the database, and when comparing CRSIds.

Collation Vs. Field

The alternative to this would be to add a db_collation to the CRSId field, which I investigated and would also work for comparisons. The advantage of the overriden Field in this case is that it ensures that the CRSId is saved into the database as lowercase, rather than just matching insensitively. This means (for example) when we serialise the Account to return from the API the CRSId is always consistently returned as lowercase.