JENKINS-7824: Change assign to field to be a drop down list of users

Hi Marc,

Here's a pull request into your branch for the changes I found to make the assignee box into a drop down list of users. I guess a pull request is probably better than you having to give me access to your branch. 

The changes to the POM are doubtless not really necessary, they are just what I found was necessary to get it all to work in my development environment, so I include them too, just in case.

Let me know any comments (or just merge in if it seems to work for you!).

Ninian