Add Irker service configuration options

### What does this MR do?

This MR makes a number of hard-coded Irker parameters configurable in the service settings: Irker server host, port, and default IRC URI. It also removes the "max recipient" limit since the recipient list is configurable only by the project owner, and it makes no sense to update the limit when it is implied in the recipient list already.

### Why was this MR needed?

The existing service assumed that gitlab.com was running an Irker daemon on `localhost` when it was not. Using Irker on gitlab.com thus did not work at all. This MR allows users to provide their own Irker daemons.

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

My main concern is whether allowing a user to specify the server/port combination would have security implications for a host. Given that HipChat and Slack allow users to do this, I didn't think this was doing anything novel.

### What are the relevant issue numbers?

* Closes #1713
* Closes #1714
* Closes gitlab-com/support-forum#139

### Screenshots

### Before

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/2eb3eb815e249e9fb669fc97ecd4f3c8/image.png)

### After

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/cceaba951c05bd3df2c842cc68046b87/image.png)

See merge request !930