Add webauthn beta badge

[nlhkabu]

Closes #5976

To be merged after #5795. I have made this as a separate PR, so that we can easily revert to remove the badges :)

Screenshots

@brainwane as per your issue, did you want me to link through to here, or are we planning to add a FAQ?

[brainwane]

@brainwane as per your issue, did you want me to link through to here, or are we planning to add a FAQ?

@nlhkabu Good question. I think we should add a "what are beta features?" FAQ item. And then we can update that FAQ with "here are the current beta features" and provide appropriate hyperlinks -- in the case of WebAuthn I need to update https://wiki.python.org/psf/WarehousePackageMaintainerTesting to give context and suggest how to test it.

[brainwane]

@nlhkabu

Here's a quick draft of an FAQ item:

Q: What does the "beta" badge mean? What are Warehouse's current beta features?

A. While Warehouse's maintainers are deploying new features, at first we mark them with a small "beta" symbol to tell you: this should probably work fine, but it's new and less tested than other site functionality. Right now our beta features are:

• In your Account Settings: WebAuthn (U2F key) support for two-factor authentication, rolled out as beta 11 June 2019.

I updated https://wiki.python.org/psf/WarehousePackageMaintainerTesting enough that it would be fine to link to in an FAQ.

[nlhkabu]

Updated to work with new UI and FAQ:

[ewdurbin]

replaced with #6027, the merge situation got a bit too wild for me to unwind.