Add a speakers page in the admin section

This is based on #270 as it shares a lot of the styling changes, so that should be merged first.

Add an additional page to show just speakers (i.e. users who have submitted sessions).
I style these using CSS where appropriate. Personally, I don't like marking fields up like this as it hard-wires display values in the code, but this is how the validation works. For this particular model, we don't (currently) use it to edit, so it is less of an issue.