Add a checkbox to the editor to set key event status

As a liveblog user, having a checkbox to set whether an individual live blog is a key event presents a more user friendly experience.

Start by locating the live blog editor and the existing key event status handling. Determine how an individual live blog's status is represented and where editor controls are rendered. Done means the editor has a checkbox for setting key event status and the chosen value is saved and reflected correctly.

Written by the indexing model from the issue text.