#862 Fixes behavior of event registration view

[kjporter]

This PR will:

• Locks position info after sign up (restricts user action to delete for consistency - removes the implied functionality that a user can edit their requested position after sign-up... this was never intended)
• Close Events Signup Registration Update #862

[iccowan]

I think this would look better if we also added disabled to the text fields for times (see the difference here between the left and the right). This will also match for when the position is assigned.