Add filtering to GraphQL API queries

[deekerno]

Now that #460 has been merged, we should investigate adding filters to GraphQL API queries. Up until now, the only supported filter has been an equivalence (=) relation. We should probably support the following capabilities:

• equivalence (=)
• comparison (<=, <, >, =>)
• set inclusion/exclusion (in, not in)
• has non-null value

Each of these would have to implemented differently for each class of ColumnType, e.g. integer, string, etc.