Improve on-boarding experience

This pull request improves the on-boarding experience for new users when 'must approve users' is enabled.

As outlined in [meta/4580/22](http://meta.discourse.org/t/can-discourse-be-configured-to-always-require-login/4580/22):

1. User creates new account
1. [Message is displayed](https://github.com/chrishunt/discourse/commit/3ad00d487269d266176db7a4db2c9a54eaf182ce)
  - "Thanks for signing up. We will notify you when your account has been approved."
  - *No email is sent*
1. Admin approves new account
1. [Message is displayed to the admin](https://github.com/chrishunt/discourse/commit/6ca00564d3a127f54ed58c34fe2a56f58dcec8b0)
  - "Approved, email sent ..... etc"
1. [User receives email](https://github.com/chrishunt/discourse/commit/10e67e5bd1e742c1f0fc6eaf9dba667978b0e42c)
  - "Welcome!! Awesome!! Click this link to activate your account"

As for the edge cases:

1. User creates account and tries to login before being approved:
  - [A message is displayed](https://github.com/chrishunt/discourse/commit/79e062eeee6e05ac9544f6a51c8cef98623494eb)
    - "Your account hasn't been approved yet. You will be notified by email when you are ready to log in."

1. User receives approval email, but never clicks the link to activate account. Then that user tries to login.
  - [A message is displayed](https://github.com/chrishunt/discourse/commit/79e062eeee6e05ac9544f6a51c8cef98623494eb)
    - "You can't log in yet. We previously sent an activation email to you at user@example.com. Please follow the instructions in that email to activate your account. Click here to send the activation email again."