accounts: replace bash with default shell

tower: https://tower.im/projects/69dc94f2e66248d5b722b7d9609b8fe7/todos/394caca5b0ed4fe4aada515ec7c1ad5b
Change-Id: I5e5a67f71cfc25f3b53ebb3b60d1468d52d8c9c2
debian: update postinst

Change-Id: Ibfc256861964f56e057e6d2a43771105e5680a42