Modularize Postgres-like data source behavior
[#62864344] User can create a PostgreSQL Data Source

 - This includes creation and display in the data sources list
 - All actions on the data source list sidebar
 - Data source browsing is not yet functional
[#62864344] Adjust Connection hierarchy

 - Logic from GreenplumConnection goes to PostgresLikeConnection
 - GreenplumConnection and PostgresConnection extend PostgresLikeConnection
 - Fixes #version for PostgresConnection