Add application setting to restrict user signups to e-mail domains

This feature was requested long ago:

http://feedback.gitlab.com/forums/176466-general/suggestions/4118466-ability-to-register-only-from-ceratain-domains

This MR is based off !253 but changed to use application settings and use wildcard strings
to give more flexibility in pattern matching. Regexps seemed overkill and prone to mistakes.

Also note that validation is ONLY done on creation to prevent breaking existing users who do not have a whitelisted domain. However, this allows a user to sign-up and change his/her email to a non-whitelisted domain.

Screenshots:

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/b312046aae03971f37f4247382971fc6/image.png)

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/94bdf3ffaf37c2e8324eff83308f81f0/image.png)

See merge request !598