As an event organiser, I should be able to associate a user with a particular speaker record on a talk. If there is only one unlinked speaker, we can assume that the organiser means that speaker. If there is more than one unlinked speaker, the organiser should also indicate which speaker this user is.
To pick a user, the organiser should start typing in a textbox and then be offered a dropdown list of matching usernames/display names. Usernames are unique across the system.
The talk will not be linked to the user's account until they approve the claim (specified separately)
This is probably part of the create/edit talk page.