Fix privilege escalation issue with OAuth external users

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/19312

This MR fixes a privilege escalation issue, where manually set external users would be reverted back to internal users if they logged in via OAuth and that provider was not in the `external_providers` list.

/cc @douwe

See merge request !1975
(cherry picked from commit 5e6342b7)

Fix visibility of snippets when searching

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/18997

See merge request !1972
(cherry picked from commit 8a197c15)

Update omniauth-saml to 1.6.0 to address a security vulnerability in ruby-saml

Updates `omniauth-saml` to bring in the new `ruby-saml` dependency that addresses [CVE-2016-5697](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-5697)

Fixes #19206

See merge request !4951
(cherry picked from commit c3a8b252)

Only show notes through JSON on confidential issues that the user has access to

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/18535

See merge request !1970

Forbid scripting for wiki files

Wiki files (not pages - files in the repo) are just sent to the browser
with whatever content-type the mime_types gem assigns to them based on
their extension. As this is from the same domain as the GitLab
application, this is an XSS vulnerability.

Set a CSP forbidding all sources for scripting, CSS, XHR, etc. on these
files.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/17298.

See merge request !1969

Remove 'unscoped' from project builds selection

This is a fix for this security bug: https://gitlab.com/gitlab-org/gitlab-ce/issues/18188

/cc @kamil @grzegorz @stanhu

See merge request !1968

fix typo making gitlab.com importing to fail

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/565

See merge request !4181

Use the relative url prefix for links in Wiki

Retry of gitlab-org/gitlab-ce!4026

@rymai !4050 solved all other problems how it looks like. I [tested](https://gitlab.com/artem-forks/gitlab-ce/commit/ff01eca7b559efa7cacf3412aa01cd8ae8a6db7e/builds) this with ruby22

Fixes #17071 



See merge request !4131

Create import data in service and fix timing issues when scheduling job

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/17401

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/17376

See merge request !4106

Fix an issue when filtering merge requests with more than one label

Fixes #15529.

See merge request !3886

Fix build notification on merge request page change even if the build status didn't change

## What does this MR do?
This MR contains a bugfix for #17357 which was introduced by !3998. The notification are now only shown on status changes, and not when switching between different merge requests.

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

## Why was this MR needed?
Because auf a bug introduced in !3998.

## What are the relevant issue numbers?
#17357

Closes #17357

See merge request !4086

Relative Links in the Wiki Are Broken

- [ ] #16568 (!4050) Relative links in wiki are broken
    - [x] Investigate issue

  
    - [x] Implementation / Fix

        - [x] Write (failing) tests for `WikiLinkFilter`
        - [x] Link to `./bar` should either get rewritten correctly or left alone
        - [x] Link to `./bar.md` should maybe get rewritten correctly (is left alone currently)
        - [x] Link to `bar.md` should get rewritten correctly


            - [x] Check if this indeed a bug

    - [x] Make sure CI is green
    - [x] Assign to endboss

    - [x] Wait for review

    - [x] Implement review feedback
    - [ ] Wait for merge


See merge request !4050

This MR never made it into 8.7.4.

[ci skip]

This reverts commit d1ba0986.

[ci skip]

Use a case-insensitive check to compare URI schemes

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/17299

See merge request !1965

Add if exists to drop command

Add `IF EXISTS` as a precaution. Related to gitlab-org/gitlab-ce!4020

See merge request !4100

Rake drop tables with cascade

See merge request !4020

Fix build notification on merge request page change even if the build status didn't change

## What does this MR do?
This MR contains a bugfix for #17357 which was introduced by !3998. The notification are now only shown on status changes, and not when switching between different merge requests.

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

## Why was this MR needed?
Because auf a bug introduced in !3998.

## What are the relevant issue numbers?
#17357

Closes #17357

See merge request !4086

Allow Redmine issue references to work as intended

Closes #14527 and #14894 

See merge request !4048

Use sign out path only if not empty

Fixes: https://github.com/gitlabhq/gitlabhq/issues/10066

See merge request !3989

Pass trusted_proxies to action_dispatch as IPAddrs instead of strings

Without this setting your own trusted_proxies does not work.

Fixes an issue introduce in: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3524

Fixes: https://gitlab.com/gitlab-org/gitlab-ce/issues/17004

See merge request !3970

Fix importer bug when throwing exceptions

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/15681

See merge request !3941

Initialize wikis on legacy projects during check

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/4173

Helps https://gitlab.com/gitlab-org/gitlab-ce/issues/15423

See merge request !3931