* Problem: `ValidatorElection` and `MigrationElection` need to inherit from a common `Election` class.
Solution: Factored the common logic out of `ValidatorElection` and moved it to `Election` parent class.
* Problem: No need to store different types of elections in their own tables
Solution: Remove `DB_TABLE` property from `Election` class.
Solution: Created the `elections` table with secondary_index `election_id`.
* Problem: `UpsertValidatorVote` can be generalized to just be `Vote`
Solution: Renamed, refactored and moved the `Vote` class to tie in with the more general `Election` base class.
* Problem: `election_id` is not unique if two elections have the same properties.
Solution: Added a random `uuid4` seed to enforce uniqueness.
* Problem: we have unused and outdated fixtures
Solution: clean up fixtures and tests accordingly
* Problem: there are still unused fixtures
Solution: remove Merlin keys
* Problem: There are unnecessary markers
Solution: remove the tendermint marker for tests
* Problem: Source files contain no license info
Solution: Add comments with SPDX license info to source files
* Python 3 files don't need # -*- coding: utf-8 -*-
