Refactor schemas for postgres

[tbantle22]

This changes how we handle the hierarchy of postgres schemas. Before for postgres databases each connection could only handle one database and the database could not be changed (because postgres does not have use [db] queries and changing the database requires changing the connection). So to simplify changes to support postgres we essentially just equated mysql databases with postgres schemas. This worked fine enough for postgres, but once we started working on doltgres, the synonym started to fail since branches are per-database and not per-schema.

This PR has a few changes to accommodate the above:

1. Move the schemas selector to the table nav below the branches selector. We now have to pass around the schemaName to different pages, which accounts for a lot of the changes
2. Allow changing the database from the top nav (this destroys/recreates a connection with the new database)
3. Postgres tables have the schema qualifier (i.e. the table name is schema.table instead of just table)
4. Rename the Schemas tab to Definitions to avoid two "schema" titles that mean different things

The new UI looks like this:

This will not work for doltgres databases yet, just postgres. That's coming next

[liuliu-dev]

lgtm!