6.0: Change Default for JUPYTER_PLATFORM_DIRS to "true"

[blink1073]

[minrk]

We'll need to make sure to migrate user config files automatically to the new locations, to avoid major disruptions to users. If we don't migrate files automatically, we should at least check, and provide a one-line migration command from the old location to the new.

We still have the migration code for the move from .ipython to .jupyter, and should update it to handle the soon-to-be deprecated Jupyter paths, too.

[minrk]

Also related to this: the platform dirs change appears to still be entirely undocumented outside jupyter-core's changelog.

It should at least be covered here and somewhere in the jupyter-core docs before the deprecation clock can safely start.

[ivanov]

I strongly oppose this proposal and think we should go in the opposite direction and remove the deprecation warning that was added in #292, at least on unix-like systems. There's no good reason to keep moving the default location for active users. Those with a preference for where their dotfiles and config directories should go have a way of speciying that.

[minrk]

#447 implements @ivanov's proposal to remove the deprecation warning.