Improve talk offer process

[jamezpolley]

• When a user clicks "Edit", use AJAX to flip them back to the right tab, don't reload the entire window.
• When a user has successfully made an offer, give them a clear unambiguous message indicating this.
• Messaging about what the fields are for and where the details will be used can be improved - eg, to clarify what the DisplayName and Contact Details are for, and where the Summary and Details will be used (and what detail should be in them).
• It's not at all clear that any "Offer" you make may/may not be accepted by an event organiser for some future event. Perhaps users should be able to indicate availability (I could give the talk in June or August, but I'm away in July; I would prefer to give this talk in July but could do it in August if neccessary).
• Need a way for users to cancel an offer (datamodel is there, just no UI)
• Admins should not be able to edit the "Consent" form, but should be able to assign an Offer to a User (making it easier for an admin to create the event on their behalf, and then just ask them to log in and give consent).