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