We were doing it most of the time, but in some cases we were clicking
the "Sign out" link instead. These actions aren't the same, just like
using `login_as` is not the same as visiting the sign in page and
submitting the form.
Some of these tests failed sometimes because the user wasn't signed in
after using `login_as`. One possible cause could be that we weren't
adding an expectation after clicking the "Sign out" link.
So using `logout` adds consistency, simplifies the code, and might
reduce the chance of these tests failing in the future (although they
might still fail in the future because some of these tests check the
database after a `visit` call).
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