Add model and data import for Legislator commitee assignments

[lazarus1331]

We should add committee assignments for Legislators. Add a new model for committees, and add a many-to-one relationship to Legislators.
This data should be available from Openstates API. Update the import_legislator command to also include the committee data.

[lazarus1331]

Check if open states has this data.

[lazarus1331]

Openstates doesn't have committee assignments for Texas.