Secure your GitLab installation with Let's Encrypt

I'd like to write a post for using Let's Encrypt with GitLab that will cover both Omnibus and source installations.

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It should:

• Be universal. Work for all distros we support and NGINX/Apache
• Use the webroot plugin to issue the certificate so that there is no downtime for existing installations

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Borrow things from:

• https://www.digitalocean.com/community/tutorials/how-to-secure-nginx-with-let-s-encrypt-on-ubuntu-14-04
• https://www.digitalocean.com/community/tutorials/how-to-secure-apache-with-let-s-encrypt-on-ubuntu-14-04

• https://webnugget.de/setting-up-gitlab-with-free-ssl-certs-from-lets-encrypt-on-ubuntu-14-04/

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I would say it could take 1-2 full days to write it, as it needs proper testing and it will depend on how much info I would put into it (include revocation method, etc.).

cc @sytses @nearlythere @ashleys