Document file upload random uuid security

This documents the current state of file uploads regarding random UUID and security.
Fixes #3569 

Thanks to @DouweM for the language.

cc/ @sytses @JobV @axil Does this look OK?

See merge request !2055

Upgrade Sidekiq to 4.x

Upgraded Sidekiq to 4.x.
More details can be found here: #3065 

After merging this MR, we will require Redis 2.8+.

@stanhu have made a MR !1888 to reflect needed changes on our documentation.

See merge request !2057

Fix editing notes on a merge request diff

Fixes #3910

See merge request !2041

Add missing changelog items

Closes #3973

[ci skip]

See merge request !2053

Reason: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2023

[Second try] Implement different Markdown rendering pipelines and cache Markdown

!1602 already got merged in bcd89a58, but it would appear the merge commit disappeared because of #3816 (or some other reason).

cc @rspeicher 

See merge request !2051

# Conflicts:
#	spec/lib/gitlab/markdown/autolink_filter_spec.rb
#	spec/lib/gitlab/markdown/commit_range_reference_filter_spec.rb
#	spec/lib/gitlab/markdown/commit_reference_filter_spec.rb
#	spec/lib/gitlab/markdown/cross_project_reference_spec.rb
#	spec/lib/gitlab/markdown/emoji_filter_spec.rb
#	spec/lib/gitlab/markdown/external_issue_reference_filter_spec.rb
#	spec/lib/gitlab/markdown/external_link_filter_spec.rb
#	spec/lib/gitlab/markdown/issue_reference_filter_spec.rb
#	spec/lib/gitlab/markdown/label_reference_filter_spec.rb
#	spec/lib/gitlab/markdown/merge_request_reference_filter_spec.rb
#	spec/lib/gitlab/markdown/redactor_filter_spec.rb
#	spec/lib/gitlab/markdown/reference_gatherer_filter_spec.rb
#	spec/lib/gitlab/markdown/relative_link_filter_spec.rb
#	spec/lib/gitlab/markdown/sanitization_filter_spec.rb
#	spec/lib/gitlab/markdown/snippet_reference_filter_spec.rb
#	spec/lib/gitlab/markdown/syntax_highlight_filter_spec.rb
#	spec/lib/gitlab/markdown/table_of_contents_filter_spec.rb
#	spec/lib/gitlab/markdown/task_list_filter_spec.rb
#	spec/lib/gitlab/markdown/upload_link_filter_spec.rb
#	spec/lib/gitlab/markdown/user_reference_filter_spec.rb

Run db:reset before db:create on CI

Should prevent build failures caused by missing migrations, such as
https://gitlab.com/gitlab-org/gitlab-ce/builds/439927

See merge request !2045

Enable Devise paranoid mode and ensure the returned message is the same
every time. This will prevent user enumeration (low impact). 

Prior to this change a user could type an email in the password reset
field and if the email didn't exist it returned an error. If the email
was valid it returned a message saying the forgot password link had been
emailed. After this change the user will receive a message that if the
email is in our database the reset link will be emailed. 

I also changed the throttle mechanism so it still works the same but
now returns the exact same message as above. Previously it would say
'You've already sent a request. Wait a few minutes'. This also allows
user enumeration, although it requires a double-check.

Related to https://dev.gitlab.org/gitlab/gitlabhq/issues/2624

See merge request !2044

Fix list with controls display

Introduced in !2025

## Before

![before](/uploads/969f021f1f97062439cc4e836f29bdf6/before.png)

## After

![after](/uploads/defd46ddbd8392625848dfd3ac067869/after.png)
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>


See merge request !2048

Skipped CI because it has already passed. Had to rebase due to CHANGELOG.

Fix inconsistency with no ssh key message

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Use YAML.safe_load

See merge request !1941

Downcase commit author email for matching with users

Closes #3854 

See merge request !1992

Add number_with_delimiter to build counts



See merge request !2046

Fix migrations for postgres on test environment

Make `be rake db:migrate:reset RAILS_ENV=test` work
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !2002

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Closes #3701

[ci skip]

Fix typos in integration docs

[ci skip]

See merge request !1999

Update init script only once

The init script is already being updated in section 6.

See merge request !2038

[ci skip]

Simplify shared User SSH key steps

See merge request !2043