Split upload view into smaller partials

This will make it easier later when we add authentication.
Add admin GUI
Refactor authentication into helper methods

The same authentication sequence will be used to verify that a user owns
an already-saved dbinbox, so we'll put the sequence into helper methods
so we can reuse it.
Move get_user to helper methods
Add ability to add access codes to dbinboxes

* Dropbox owners can add an access code

* Visitors must enter the access code before uploading

* We let Dropbox authenticate the user each time they use
  the admin tools so we don't have to manage password resets
Allow the user to clear the access code
Use a local var instead of an instance var
Move admin tools onto their own page

* Switch the "Admin" link from a button to a link in the upper-right
* Consolidate the upload page into a single view