v0.6.0 - when we got a backend datastore

Add schema migration file for create users table in chirpy db

Add util.Censor to cleanse the profane chirps

Refactor API reponses to goserver/internal/util sub-package

refactor: add internal sub-packages, update go.mod, main.go

Bugfix/Handle empty chirp inputs in /api/validate_chirp

Merge pull request #1 from FlyingJ/add_validate_chirp

Add validate chirp

Modify fmt.Sprintf for /admin/metrics to use multiline quote