Add new forms.py with LoginForm to source_app
Add new forms.py with LoginForm to source_app

This LoginForm uses Flask-WTF's validators to validate the the
codename field contains input, is not longer than the max codename
length to avoid DoS, and matches only the expected characters via
regex.

garrettr/securedrop,heartsucker/securedrop,conorsch/securedrop,micahflee/securedrop,conorsch/securedrop,micahflee/securedrop,conorsch/securedrop,garrettr/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,heartsucker/securedrop,micahflee/securedrop,heartsucker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,garrettr/securedrop,conorsch/securedrop,micahflee/securedrop,ehartsuyker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop