Add democracy migration and fix contracts migration (#12)

* log current blocknumber during migration

* improve phragmen migration logs and docs

* add combined democracy migration

* re-add ReferendumInfo hasher migration and fix compile issues

* add more logging for democracy migration

* add migration from democracy weights PR https://github.com/paritytech/substrate/pull/5828

* log key used for PublicProps

* migrate preimages

* correct logging and rename variables

* add block number for block hash migration

* log decode errors

* add decode error log label

* remove extra logging

* re-add contracts migration

* remove weight todos and just assume MaximumBlockWeight

* fix compile errors

* replace some todos with comments

* log decode error

* fix contracts migration

* remove grandpa authority migration