Move current user to the top of the list in author/assignee filter

### What does this MR do?

This MR puts the current user at the top of the user list in the author/assignee filters.

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

Is it best to do this in the JavaScript as it is in this MR, or are we better off adding an argument to the `/users` API to put the current user on top?

Also, I elected to put the current user after "Any" and "Unassigned"--it looked better. Do you agree?

### Why was this MR needed?

If you're trying to look for "Me" in the list, often you have to scroll down a long way to find yourself. Using the filter to find yourself is yet another step requiring the keyboard, and it's easy to forget that feature is available.

### What are the relevant issue numbers / [Feature requests](http://feedback.gitlab.com/)?

#1321 and #1396

### Screenshots

Before:

![Screen_Shot_2015-04-08_at_11.18.44_PM](https://gitlab.com/stanhu/gitlab-ce/uploads/4c28ae5a3a51a8329c806ecfbc549071/Screen_Shot_2015-04-08_at_11.18.44_PM.png)

After:

![Screen_Shot_2015-04-08_at_11.20.06_PM](https://gitlab.com/stanhu/gitlab-ce/uploads/ff3b8bdf38467236edbcc29401cd1235/Screen_Shot_2015-04-08_at_11.20.06_PM.png)

See merge request !505