Use cobra to implement superadmin and other administrative commands, versus running the server

[calvinchengx]

It is rather inconvenient to access the migration cli as a separate app from our main server app.

Let's unify them with spf13/cobra so we can issue consistent command line commands from our project root.

[calvinchengx]

We choose spf13/cobra over urfave/cli because cobra supports subcommands.

[calvinchengx]

• [x] by default, run our API server
• [x] command to create database user and database
• [x] command to create schema
• [x] command to initialise schema migration table
• [x] command to run all available migrations
• [x] command to run migrations up to the target one
• [x] command to revert last migration
• [x] command to reset all migrations
• [x] command to print current db (schema) version
• [x] command to set db (schema) version without running migrations
• [x] command to create superadmin user