Provide a "Submit a Talk" link for unauthenticated users

[jrrickerson]

While we intend for users to be required to register to submit a talk proposal, simply excluding the link to the submission page if the user is not authenticated is likely going to be confusing to new users - if they don't see any link, they may not realize you can submit a talk proposal at all, and if any descriptive text on the site mentions submitting a talk, they won't know where to go to find that.

Allow the Submit link to be visible to unauthenticated users, but if clicked, have the Submit template inform the user that registration is required and provide a link to the Registration page, which should eventually redirect them back to the Submit page.