The sessionList code has no dependencies on anything else in
loginDialog.js so move it to its own file.
This is part of the greater reorganization effort to clean up
the login dialog / unlock dialog situation.
Right now the GDM and unlock code use a mish-mash of files
from ui/ and from gdm/.
This commit consolidates all the files into ui/auth as a
first step toward cleaning up the duplication of code between
the two features.