:heartpulse: Add subdomain field to account form. :heartpulse:
Create the subdomain field to account table. :herb:
Permit access to subdomain :computer:
:snowboarder: Redirect to account's subdomain!
:100: Share sessions under example.com to pass the spec.

By default Rails store flash message in the session.
And one session is scoped to specific domain.
example & test.example.com will be 2 separate session.
Then subdomain will not receive flash messages.
To fix this, we share root & subdomain use the same session store.