Security advisory for Logjam vulnerability

This post addresses the Logjam vulnerability by detailing its impact on GitLab and GitLab.com. For self-managed installations, it provides instructions on how to generate 2048-bit DH groups using `openssl dhparam` and configure Nginx via `gitlab.rb` for both omnibus packages (version 7.11.0 and up, and prior versions) and source installations. For GitLab.com, it states that 1024-bit DH groups are in use and that enabling 2048-bit DH params is being investigated due to compatibility issues with older Java-based clients.