Make `bokeh` a hard dependency?

While it's not strictly necessary for running computations, the vast majority of `distributed` users probably want access to the Dask dashboard when they create a cluster. Currently, `bokeh` is an optional dependency for `distributed`, so with a simple `pip install distributed`, the dashboard isn't available. My current thinking is that adding `bokeh` as a hard dependency would do more good than harm. What do others think?

cc @jacobtomlinson @fjetter

EDIT: I've not fully looked into package sizes yet. Maybe `bokeh` is large