Extend unit admin interface to allow updating name to match a previous year

[hancush]

Related to #553.

Right now, if there's a typo or inconsistent formatting in a unit's name, that unit isn't linked year over year, and its payroll must be reuploaded with the correct unit name in order to be linked. That's a bummer! Update the unit admin interface so, if an employer name is changed (and perhaps a box is checked to confirm you want to merge), and the updated name matches an employer we already have on hand, those employers are merged into a single entity and their data is linked year over year.