feat(*): Add a default database setting

Only set for development purposes.
feat(common): Add `common` app to INSTALLED_APPS
feat(common): Add a "login required" view mixin
feat(common): Add an "anonymous only" view mixin
Merge pull request #3 from brasilcomvc/feat/auth-view-mixins

feat(common): Add auth view mixins
feat(acc): Introduce the User model
feat(acc): Introduce a bare Signup page