test: remove hard-coded sequelize dialect

[LoneRifle]

Problem

While GoGovSG uses a variety of postgres-specific queries and features,
the codebase still mostly talks to the database via Sequelize. To facilitate
integration testing, we will have to use something other than postgres,
eg an in-memory embedded database like SQLite.

Solution

Remove the hard-coded dialect in Sequelize options and let the dialect
be driven from the database URI. This allows us to easily make use of
other database implementations where needed.

Considerations

If the hard-coded dialect is kept, integration tests would be limited to
running on CI or an elaborate development environment. Integration tests
could run on a developer environment by mocking the postgres client, but
this is tedious and does not account for potential mistakes in SQL query
syntax or behaviour

[liangyuanruo]

This looks like a pretty fundamental change. What is the impact to system behavior and underlying semantics if the dialect is removed?

[LoneRifle]

Per description - no impact to system behaviour, though dialect is now determined by the database URI, which prefixes the protocol (eg, mysql, postgres, sqlite etc)