Rethink speaker leaderboard

[marcoroth]

#208 introduced a speaker leaderboard. While these are cool metrics I think it shouldn't be a seen as a "competition" or "leaderboard" on who has given the most talks.

Maybe we can pack this information differently. For example, by providing a "sort" column on the /speakers page and having the options "Name ASC/DESC, Talks Count ASC/DESC" etc. to convey the same information but with a different intent.