models: json objects are stored as strings in json columns

[mdonadoni]

Many database tables use JSON columns, but sometimes JSON objects are instead stored as strings.

We should check whether JSON columns are actually needed and/or if they are properly populated.

For example, env_vars and cmd columns of Job are not properly populated, see source code.

We would also need to prepare migration scripts to fix the issue for REANA instances that are already deployed.

[mdonadoni]

See https://codimd.web.cern.ch/-wya8pNTQUuc1Cl8vhjBZg