[0.12.1] Write and disseminate point release messaging

[eloquence]

Part of #4260. We don't do pre-release announcements for point releases, but we still need to do the needful:

• Release notes posting on SecureDrop.org
• Tweet from @securedrop
• Bulk announcement to support portal members

[eloquence]

First draft up: https://docs.google.com/document/d/1EuWBaQHjNmdHuRF5EkEPdH7-h51DnkcklhByJ64DlRg/edit#

[eloquence]

All done.

Blog post:

• https://securedrop.org/news/securedrop-0121-released/

Tweets:

• https://twitter.com/SecureDrop/status/1108507830342963200
• https://twitter.com/SecureDrop/status/1108508202113490944