Clean up EIA-861 tables

[zaneselvans]

We pulled the "interim" EIA-861 tables into the database in #2265 / #2403 but they've definitely got some rough edges.

- [ ] #2414 
- [ ] Improve EIA-861 field & table descriptions.
- [ ] Use coding table to standardize `balancing_authority_code_eia` values in all EIA-861 tables.
- [ ] Create coding tables for EIA-861 columns that currently have ENUM constraints.

Out of scope

• Harvesting utility_id_eia and balancing_authority_id_eia values.
• Creating a balancing_authorities_eia861 entity table.
• Completing the normalization of the EIA-861 tables.