Add PGP/GPG integration: verify signatures on commits and tags & more

UPDATE: This issue has been split into three parts:

1. PGP/GPG Integration - display/highlight signed commits and tags
2. PGP/GPG integration - Signature
3. PGP/GPG Integration - reject un-signed commits with hooks

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As requested in the deprecated feature request system I want to request that PGP verification should be integrated in GitLab: The Webapp sould

1. display/highlight signed commits and tags similar to git log --show-signature
2. display some basic information about the signature (Name, E-Mail, Key ID, Date)
3. GitLab EE git hooks can reject un-signed commits (see git push-Manpage)

Especially the 3rd point is explained in this article well.

Using public keyservers to fetch keys will be discussed in issue #19240 (moved).