De-duplicate "Owner" and "Maintainer" roles

[di]

Currently on pypi-legacy, it's possible for a user to have both the "Owner" and "Maintainer" roles.

However, an "Owner" can do everything a "Maintainer" can do, so it doesn't make sense for a user to be able to have both roles.

In the short term, we decided to add a hack to hide any "duplicate" roles in #2705, and remove the ability to add multiple roles for the same user in Warehouse.

Once pypi-legacy is shut down, we can run a migration which removes any of these "duplicate" roles., and remove this hack.

[brainwane]

I think I'm unclear -- what can an Owner do that a Maintainer can't do? I am probably going to open a documentation issue to put that info somewhere users can get at. Thanks.

[di]

@brainwane The following information box is included on the 'Collaboration' page where a user has the ability to change these roles:

I think that should probably suffice in terms of documentation, but it probably wouldn't hurt to duplicate it elsewhere.

[di]

Actually, this should probably get revised, because Owners can do other things too now (like delete files, releases, and the entire project).

[di]

This was causing the 503 error experienced in #4728, seen here: https://sentry.io/python-software-foundation/warehouse-production/issues/677429114/