Rename sanitize in check task

This is to prevent the name clash between `sanitize_helper.rb` method sanitize and the method used to check if env variable is set in rake task.

See merge request !1336

Differentiate system notes

Remove unused has_gitlab_shell3? method

Use blob local instead of instance.

Disable / hide MR edit blob button if cannot edit.

Update gemnasium-gitlab-service to version 0.2.3

Fix dev user seed: ID was used twice.

Add tests for tree edit routes

Reduce the size of Docker image

Update the Omnibus package in Dockerfile

transfer error handler

[BUGFIX] Invalid branch in comparison

Fix Rake tasks doc README: add top level h1

Add theme type css class

Add link to OmniAuth config sample for Omnibus

Added a link to the OmniAuth config sample for Omnibus installations.

See merge request !1335

Update Documentation for 7.6 release

Updated the update and install documentation for the 7.6 release.

See merge request !1334

and link to missing to features.md.

Credentials in readme

See merge request !1332

Dev docs in gdk

Also see https://gitlab.com/gitlab-org/gitlab-development-kit/merge_requests/31

See merge request !1331

update order of upgrade guide

1) Add `--no-install-recommends` option to `apt-get install`, this
   avoids lots of (~30MB) unnecessary packages.
2) Add `ca-certificates` package for `wget` fetching stuffs from Amazon
   S3.
3) There is no need to run `apt-get clean` for an image derived from
   official Ubuntu since they already cleaned (see also:
   http://goo.gl/B2SQRB) all the garbages produced by `apt-get`.
Signed-off-by: kfei <kfei@kfei.net>

From 7.5.2 to 7.5.3.
Signed-off-by: kfei <kfei@kfei.net>

Add kerberos to gemfile

See merge request !1330

Add documentation for the Sidekiq MemoryKiller

We already merged the code, and we are already using it on gitlab.com. This MR adds documentation.

See merge request !1325

Update changelog 7.6

See merge request !1327