[0.10.0] Write and disseminate release announcement

[eloquence]

Part of release epic #3849. Provided we're good to go, we should disseminate a release announcement through the usual channels on October 23, 2018:

• Incorporate final changelog additions, including security fixes
• Write release credits
• Post to SecureDrop.org blog
• Announce to Support portal members
• Post to Twitter

See #3843 for messaging background.

[eloquence]

Ready for review here:
https://docs.google.com/document/d/1STDTd1aWSTXJTO5XsVOwL3-KHqf1elQXy17AYAs38pk/edit#

@emkll has signed off on security-related additions. L10N credits are derived from #3895; I've omitted them for languages where Kushal is the only person credited (he's credited separately as the coordinator). Commit credits are derived from the commit log in the current RC branch.

[eloquence]

@redshiftzero approved via Slack.

[eloquence]

All done. Public notes here:
https://securedrop.org/news/securedrop-0100-released/

Tweet here:
https://twitter.com/SecureDrop/status/1054844337224212480