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:

See merge request !291
Make Irker service check supported_events
63178a9509 added the supported_events check for services. Since f84b7eef3f was just added, we should adjust it accordingly.
See merge request !367
Supports four different event types all bundled under the "note" event type:
- comments on a commit
- comments on an issue
- comments on a merge request
- comments on a code snippet
1) Adds a DB migration for all services to toggle on push, issue, and merge events.
2) Upon an issue or merge request event, fire service hooks.
3) Slack service supports custom messages for each of these events. Other services
not supported at the moment.
4) Label merge request hooks with their corresponding actions.
Irker is a gateway which sends IRC messages on git updates. This new
service provides an interface to this gateway, integrated in Gitlab, for
each updates.
As per the guidelines, this commit adds the new feature in the
CHANGELOG, tests and documentation.
See http://www.catb.org/esr/irker/
Strips the refs/heads/ if any to get just the branchName out of a :ref
e.g.: `refs/heads/feature/newProfile` -> `feature/newProfile`
The TeamCity buildQueue POST data require the branchName attribute
to contain just the branch name (just xyzbranch and not the full
ref in the refs/heads/xyzbranch). With the refs/heads/xyzbranch the
build is triggered and looks like it is working however it always
ompiles the default branch (or master).
Test settings for issue tracker service
This adds minimum logging and response when using "test settings" button when the service is enabled.
See merge request !1506