* Allow AssetLinks to be used in place of Assets in the Transaction Model and enforce `Transaction.transfer()` to only take an AssetLink
* Remove AssetLink's inheritance from Asset
* Remove id from the Asset model
* Fix get_txids_by_asset_id query for rethinkdb after removing asset's uuid
Because `CREATE` transactions don't have an asset that contains an id
anymore, one way to find all the transactions related to an asset is to
query the database twice: once for the `CREATE` transaction and another
for the `TRANSFER` transactions.
* Add TODO notice for vote test utils to be fixtures
* Update asset model documentation to reflect usage of transaction id
* Fix outdated asset description in transaction schema
