Newsletter updates

#### https://bugzilla.mozilla.org/show_bug.cgi?id=1110944

Update the success message text to be more generic. Everyone will receive an email but if you have an FxA then you will receive a "Welcome" email and confirmation is not required.

#### https://bugzilla.mozilla.org/show_bug.cgi?id=1109280

Remove the manage account link from account settings, it isn't very helpful and they can manage their subscription from an email.

![screenshot 2014-12-18 09 51 05](https://cloud.githubusercontent.com/assets/211578/5491053/bfc5bc9c-869b-11e4-8e25-9f031d1f6519.png)
> Desktop, `Newsletter` section removed entirely.

<img src="https://cloud.githubusercontent.com/assets/211578/5491054/bfcae956-869b-11e4-86b5-208b6ff4ae8b.png" width="400">
> Mobile, manage link removed.