[0.12.0] Write and disseminate release announcement

[eloquence]

As part of #4124, we should disseminate a release announcement through the usual channels on February 26, 2019:

• SecureDrop.org blog
• Support portal members
• Twitter

We need to emphasize that this release will be the first with support for Ubuntu 16.04 and reinforce the preparatory steps people should take prior to the upgrade.

[eloquence]

Draft messaging here: https://docs.google.com/document/d/1uPTzg55gqj9EePgYEZeS7YotrKKe07bv4IIWgKuU5O8/edit#

Main differences with pre-release announcement:

• References new language support
• Volunteer credits
• Explicit pointer to upgrade guide

Before we can distribute the messaging, we need to ensure that the docs link works and the /xenial-upgrade redirect points to an appropriate location.

[eloquence]

All done. Blog post:
https://securedrop.org/news/securedrop-0120-released/

Tweet(s):
https://twitter.com/SecureDrop/status/1100604439419744256