users should remove former Go installation folder

## What does this MR do?

It ensure users installing or updating from source will remove their former Go installation folder in `/usr/local/go`, to avoid raising errors that could lead to new issues being created.

## Are there points in the code the reviewer needs to double check?

1. The `rm -rf` should not be verbose if folder does not exists.
1. May a user have some kind of configuration files in this folder ?

## Why was this MR needed?

I did faced the error, that led me to the solution stated here : https://gitlab.com/gitlab-org/gitlab-workhorse/issues/29#note_4155517

gitlab-org/gitlab-workhorse#29

See merge request !6602

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]