rails-observer dependency required for TopbarSweeper
I erroneously removed the explicit rails-observer dep in #14434, but the sweeper functionality used by TopbarSweeper is provided by the rails-observer gem, so the dependency should be restored. (At least while TopbarSweeper exists; I think the remainder of the token refactoring in #14434 remains beneficial.)
Currently rails-observer is installed as a dependency of the audited gem, so it does still work.
fixes #18929 - add rails-observers dep for TopbarSweeper
0443e85 was in error in removing the dependency, as this sweeper
functionality is also provided by rails-observers.