Add support for listing PostgreSQL databases

[Dave Shoreman]

Could be done per-driver or as a single aggregated list, with the latter probably needing more faffing about on the Vue side.

• Potential API route might be GET /api/databases/{:driver} where driver defaults to 'mysql'
• Connection and dbal methods should be injected in constructor
• Driver selection could be either as a sidebar submenu, or a dropdown on the page
• In the list, icon should match that of the driver vendor