Sign Packages

Update https://gitlab.com/gitlab-org/omnibus-gitlab now that we have merged omnibus!7 (merged)

This adds:

• Signing of RPMs (rpm --addsign)
• Signing of DEBs (debsigs format)
• Necessary changes to get the GnuPG keys in place for signing.

Relations:

• Related to #2537 (closed)
• Related to #1054 (closed)
• Replaces !922 (closed)
• Replaces !1718 (closed)
• Replaces !1752 (closed)
• Implements omnibus!7 (merged)

TODO

• Merge this
• Implement new keypair
  • Generate new keypair
  • Upload to appropriate bucket
  • Change/add values for GPG_PASSPHRASE and SECRET_AWS_*
  • Upload pubkeys to PackageCloud repositories
  • Write documentation for users to enable the checking of package signatures
• Activate in production branches