Allow data source actions to read some information from the retcon database

**WIP - DO NOT MERGE**

Some data sources will require access to information from the retcon database to, e.g., translate references to related entities. This pull request 

Done:

- Change run operation so that DataSourceAction is evaluated from within the RetconHandler monad.

Todo:

- Update Action monad so that it carries the data store in an additional reader component;

- Implement the additional read-only operations for the Action monad.