Improvements to dialogues list presentation

For your consideration, here are a few commits that help improve the presentation of the dialogues list when neither participant in a particular dialogue is the viewing user. For example, in a course with more than one teacher, the participants of a dialogue become unclear in the dialogue list if the student initiated the dialogue. In our institution the preference is to show the non-teacher, but in a student-initiated dialogue it is the teacher who is listed for other course users with mod/dialogue:manage permission rather than the student.

Screenshot:
![dialogue](https://f.cloud.github.com/assets/523647/59191/e69aefb0-5ba8-11e2-8866-d187b0c2fa5d.png)