Add a speaker details page and link to it from the sessions

This pull request adds a speaker page, so that the details of an individual speaker can be shown/linked to. This page is linked to from each session.

To do this, I've extracted a partial view for showing the speaker details, and refactored some code in the controllers.

I've also styled the speakers page to be consistent with the sessions page so that a user's own profile will be highlighted and have a link to edit it.