Fix co_m_p_talk_proposal_fields in registration

Fix co_m_p_talk_proposal_fields partial in registration: move the <fieldset> and <legend> tags into the partial, fix nesting inside the partial.
Add a uniqueness validation to Participation
Simplify conference selection fields
Move methods for attributes from params to init

Move route handler methods for getting model attributes from params to init file.
Simplify obtaining conference list from params
Nullify empty attributes more uniformly (style)
Simplify and fix registration form and actions

Make registration form and actions behave more similarly to editing participants by organisers.