Migration strategy for proxy model permissions to use their own content type

[Arthur Rio]

Hi all,

I have been working on a 9 years old ticket that I'd like to close once and
for all. The outstanding question is about the migration path to choose in
order to update existing proxy model permissions. I have explained three
different approaches I can think of in the pull
request: https://github.com/django/django/pull/10381#issuecomment-435534644.

I have implemented the first option and wrote the release notes explaining
what to expect for users upgrading:
https://github.com/django/django/pull/10381/files#diff-1f22a5c1d6164c6de8defb36f3829138.

Carlton, who has been the main reviewer, suggested to contact the mailing
list for further review in order to catch any red flag and make sure we
have general consensus on the chosen approach.

Regards.

Arthur