Fix AssignmentPaper and Mail schemas

[sente]

Why is it we're using sednerId in these two schemas while we use senderId elsewhere?

stu@redmac ~/code/OLE/BeLL-Apps (dev) $ git grep senderId |grep /models/

app/MyApp/app/models/Invitation.js:            senderId: 'Text',
app/MyApp/app/models/request.js:            senderId: 'Text',
app/nation/app/models/request.js:            senderId: 'Text',
attachments/earth/app/models/request.js:            senderId: 'Text',


stu@redmac ~/code/OLE/BeLL-Apps (dev) $ git grep sednerId |grep /models/

app/MyApp/app/models/AssignmentPaper.js:            sednerId: 'Text',
app/MyApp/app/models/Mail.js:           sednerId: 'Text',