Add support for external wikis

## What does this MR do?
This MR adds the possibility to replace the link to the internal wiki of gitlab with a custom link.
Currently this is realised as a service.

## What Use Case does this MR solve?
In my Company we already have a wiki System (Confluence).
We have a policy to use the existing wiki, so we can't switch to the internal wiki Gitlab provides.
This currently only leaves us two choices:
1. Disable the gitlab wiki. That means we completly loose the connection between wiki and code from the gitlab ui.
2. Create a simple wiki page with a link to our external wiki and hope that no one uses the internal one.

Both solutions are not really good.
So what can be done to improve the situation while making it as easy as possible for new developers to access both, wiki and gitlab?
Replacing the wiki link kinda like the JIRA integration replaces the issues link looks like a good first step to me. :)
This can probably be extended later to completly prevent access to the internal wiki (currently that's still possible if you know the link) or maybe to check if the link really points to a wiki.

## Screenshot:
![external_wiki_service](https://gitlab.com/uploads/gitlab-org/gitlab-ce/89b27cf068/external_wiki_service.png)

See merge request !291

More actively use css variabled to prevent colors duplication

See merge request !1715

Replace linux with gnu linux

To recognize the work of Dr. Stallman.

See merge request !391

Update Grack.

See merge request !1712

Update omniauth-ldap.

See merge request !1713

Update installation and upgrade documentation

See merge request !1714

Fix service UI

There was a small bug in the service UI that caused an empty 'Trigger' label to be shown. After this change the 'Trigger' field is not shown when only one event type of supported.

Before:
![Screen Shot 2015-03-16 at 7.40.31 PM](https://gitlab.com/uploads/dblessing/gitlab-ce/0457a0b918/Screen_Shot_2015-03-16_at_7.40.31_PM.png)

I think in the future there are other things we can do to enforce at least one selected event type and also show the user what the singular event type is if only one is supported. That will take time to work out and this is definitely acceptable for the time.

See merge request !395

Delete deploy key when last connection to a project is destroyed.

Addresses #1959.

See merge request !1710

Subscription to issue/mr

Fixes #1911 and #1909

![joxi_screenshot_1426601822159](https://dev.gitlab.org/gitlab/gitlabhq/uploads/53021bc5783271322ab2dfba7598eaa3/joxi_screenshot_1426601822159.png)

![joxi_screenshot_1426601836423](https://dev.gitlab.org/gitlab/gitlabhq/uploads/244ff360fbd6f30980f8dad699400814/joxi_screenshot_1426601836423.png)

See merge request !1702

Fix invalid Atom feeds when using emoji, horizontal rules, or images

This is a fix for issues #880, #723, #1113.

Markdown must be rendered to XHTML, not HTML, when generating summary content for Atom feeds. Otherwise, content-less tags like *img* and *hr* are not terminated and make the Atom XML invalid. Such tags are generated when issue descriptions, merge request descriptions, comments, or commit messages use emoji, horizontal rules, or images.

To pass this option through from the relevant Haml templates to the proper place in the `gfm()` method, a new method `gfm_with_options()` is introduced. It reuses the options dictionary passed to `markdown()` and interprets options `xhtml` and `parse_tasks` from it (the latter was a convenient replacement for `gfm_with_tasks()`). `xhtml` is already interpreted by Redcarpet::Render::HTML, but that alone was not sufficient, because the post-processing in `gfm()` would convert its XHTML tags back to HTML.

I found no way of passing additional optional options to the existing `gfm()` method without requiring updates to existing callers and without getting in the way of the existing optional arguments, but maybe someone who knows more about Ruby than I can think of one.

Thorough review appreciated since this is the first time I have used Ruby.

See merge request !344

This reverts commit c42262b4, reversing
changes made to c6586b12.

Change permissions on backup files

Use more restrictive permissions for backup tar files and for the db, uploads, and repositories directories inside the tar files.  See #1894.  Now the backup task recursively `chmod`s the `db/`, `uploads/`, and `repositories/` folders with 0700 permissions, and the tar file is created as 0600.

cc @sytse

See merge request !1703

Documentation about unicorn settings.

Helps with https://news.ycombinator.com/item?id=9213180

See merge request !1704

Relative sematic versioning

Note to indicate that semantic versioning is not absolute or allows you to fly blind.

See merge request !286

Handle null restricted_visibility_levels setting

Fix a 500 error when the `restricted_visibility_levels` setting is null in the database - see #2134.

See merge request !1705

Fixes issues #880, #723, #1113: Markdown must be rendered to XHTML, not HTML, when generating summary content for Atom feeds. Otherwise, content-less tags like <img> and <hr>, generated when issue descriptions, merge request descriptions, comments, or commit messages use emoji, horizontal rules, or images, are not terminated and make the Atom XML invalid.

Dependency to documentation

See merge request !1707