Use image proxy to mitigate stealing ip addresses

Closes #2812

See merge request gitlab/gitlabhq!2926

DNS Rebind SSRF in Kubernetes Integration

See merge request gitlab/gitlabhq!3345

User images and videos will get proxied through
the Camo server in order to keep malicious
sites from collecting the IP address of users.

Kubeclient uses rest-client. We hack into to access the net/http object
so that we can patch to connect to the resolved IP + set
hostname_override.

Add specs for discord. The discord integration also uses rest-client, so
since we patched rest-client, spec that the DNS rebinding protection
works

- Add mail interceptor the signs outgoing email with SMIME
- Add lib and helpers to work with SMIME data
- New configuration params for setting up SMIME key and cert files

The circuit breaker itself got removed a while ago, when that happened
some parts got left behind. Using grep old stale settings and
validations were found and are now removed.

Storing this key in secrets.yml was a bad idea,
it would require users using HA setups to manually
replicate secrets across nodes during update,
it also needed support from omnibus package

* Revert "Generate Let's Encrypt private key"
  This reverts commit 444959bfa0b79e827a2a1a7a314acac19390f976.

* Add Let's Encrypt private key to settings
  as encrypted attribute

* Generate Let's Encrypt private key
  in database migration