Don't lock out admins who are impersonating

*THIS DOES NOT* actually let you change entries, b/c of its
before_update hook in Entry. That needs to be figured out separately
Try to avoid the error caused by nil timesheets
Add a new updated_by attribute to entries. 

Appropriate b/c of our new god mode. Also will make it easier to unlock
the entry validations when you're an admin
data migration to default the prior
Allow admins to impersonate and set locked entries

Squeezes in the logged in user into every entry via params hacking
(Really gross way of doing it. :shrug:)

This means that the value will be null when the user creates a timesheet
but does not update it, which i think is actually meaningful for 
tracking authorship. They just hit "next" they didn't necessarily mean
to update a thing.