Move reply by email docs to a new location

## What does this MR do?

Move reply by email docs to a new location.

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/3349

## Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

- [ ] Make sure the old link is not removed and has its contents replaced with a link to the new location.
- [ ] Make sure internal links pointing to the document in question are not broken.
- [ ] Search and replace any links referring to old docs in GitLab Rails app, specifically under the `app/views/` directory.
- [ ] If working on CE, submit an MR to EE with the changes as well.

See merge request !6517

Fixes Go 1.5 compatibility.

Update "Installation from source" guide for 8.13.0

Following the steps from https://gitlab.com/gitlab-org/release-tools/blob/master/doc/release-candidates.md#creating-rc1.

See merge request !6565

With the introduction of repository storages validations it becomes
necessary to add the flag `SKIP_STORAGE_VALIDATION` to the gitlab-shell
install command, since that command will create the storage paths

[ci skip]

Signed-off-by: Rémy Coutable <remy@rymai.me>

[ci skip]

Fixes rejected API git archive extensions; improves Sentry errors.

This adds (basic) Sentry support.

Security and safety improvements for gitlab-workhorse integration

Companion to https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/60

- Use a custom content type when sending data to gitlab-workhorse
- Verify (using JWT and a shared secret on disk) that internal API requests came from gitlab-workhorse

This will allow us to build features in gitlab-workhorse that require
more trust, and protect us against programming mistakes in the future.

This is designed so that no action is required for installations from
source. For omnibus-gitlab we need to add code that manages the shared
secret.

See merge request !5907