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).
Oh, no, I meant the whole title string. I hadn't noticed it was in the `_Layout.cshtml` file, though, so I thought that this was going to be duplicated. Obviously that won't actually be the case, so let's forget I said anything :wink: