Prompt users to set a name after they propose a talk

Some users who get talks accepted have only an email address as a name. This can happen at least two ways:

1. They sign in or created their account with email magic link
2. They were added as a co-speaker by someone with an account (the secondary users may have never signed in at all)

There may be other ways.

After proposing a talk, if any speaker has an email address as their name, we should show a persistent flash message asking them to set a name (we do something similar with demographic survey).

For the logged-in user themselves, they can fill out the user profile/speaker bio to do this.

For co-speakers, we may need a new form where the logged-in user can set a name for co-speaker. (We may need to think about abuse vectors here? In practice, we don't seem to have had abuse of the co-presenter feature, as far as I can tell)