Add reason when removing attendees

[golenkovm]

Need to be able to add reason when removing attendees.

Potentially, this could be done via adding a new field below "Suppress email notification" (or via popping up an input field on Remove click) which value would be passed through the facetoface_user_cancel call (new optional param) and stored in mdl_facetoface_signups_status.note column.