Time-limited sandbox instance with clone of PyPI DB for testing

[brainwane]

Over the next ~3 months, as we hit milestones on our development roadmap, we'll be asking various groups to test Warehouse. @ewdurbin suggested we consider creating a new instance of Warehouse -- separate from the current TestPyPI -- with a clone of the current database, useful for package maintainers/owners who want to test destructive actions in a familiar environment (without causing too much confusion).

We would do this probably sometime this month or in early March, and then -- I presume -- take it down after we shut down Legacy PyPI.

Reference to other related issues:

#720 suggests: run TestPyPI, but add a configuration option such that we run Test PyPI with a test "mode" on that allows the package maintainers/owners to re-upload (as we do not allow on the main PyPI).
#726 suggests: add a two-phase (staged) upload flag to PyPI so package owners can choose either to have artifacts immediately published upon upload, or to allow the owners/maintainers to preview them first before publication.
#2286 suggests: have a sandbox instance (either TestPyPI or some other instance) available that is entirely for test packages which would regularly get wiped. (#2286 (comment) suggests that running multiple simultaneous instances might not be the desired approach.)

[brainwane]

I asked @ewdurbin about this today and his take was: this setup would be pretty complex, and given the time constraints we're under, we will probably instead just use (and suggest our testers use) Test PyPI as is.