Warehouse: Add admin ability to reassign project to another user

[dstufft]

Legacy PyPI allowed admins to take over an existing project, but Warehouse doesn't have a mechanism to allow an admin to re-assign a project to another user.

[nanuxbe]

@nlhkabu I'm going to work on this during the ep18 sprints.

The plan is to add an expandable section at the bottom of the project's "view" page in the admin similar to "Delete Project" or "Set Upload Limit"

[di]

Since a project might have multiple owners, it would be ideal if this feature allowed us to add/remove roles for a given project.

[di]

(Also probably worth mentioning that the users in the dev DB with admin access are dstufft, ewdurbin and di)

[nanuxbe]

So far...

Both collapsed:

Both expanded:

[nlhkabu]

Can we please make the text:

"To delete role "blah" for "blah", type the role name:

• remove placeholder ;)