Gitlab Geo:  Authenticate with OAuth

This changes the way Geo (#76) nodes authenticates, to remove the requirement of session sharing between nodes, as described here: #366

# Checklist

 - [x] When a secondary Geo node is created an OAuth Application is created associated to it
 - [x] When a secondary Geo node is removed the associated OAuth application is also removed
 - [x] When a user tries to login it must be redirected to the primary node using the OAuth workflow
 - [x] The user must be redirected to the "first accessed url before authentication" (as expected)
 - [x] User should be able to logout from a secondary node
 - [x] It displays authentication errors when OAuth application is not present
 - [x] It handles authentication errors when OAuth flow fails
 - [x] Solution for what to do when/if OAuth application is removed

See merge request !237

Allow SSL verification to be configurable when importing GitHub projects

Closes gitlab-org/gitlab-ce#14190

See merge request !260

to the database

ES fix exception on branch removing

fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/382

See merge request !259

ES: Don't index newly created system messages and awards

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/381

See merge request !258

CE-to-EE merge

See merge request !256

Change all instances of "web hooks" to "webhooks"

See merge request !3169

[ci skip]

[ci skip]

OCD hooooooooo!

[ci skip]

Fixes broken layout of web editor page on mobile

Fixes #13987 

<p><img src="https://gitlab.com/gitlab-org/gitlab-ce/uploads/3a58e7b576c25d5929bc1cdcbe163161/Screen_Shot_2016-03-10_at_1.13.19_PM.png" alt="Screen_Shot_2016-03-10_at_1.13.19_PM" width="487"></p>

See merge request !3167

Adds mention of Redcarpet as Markdown parser in docs

It makes it easier for users to find the source of issues related to
Markdown parsing.

[ci skip]

See merge request !3168

[ES] Minor fix



See merge request !255