Placeholder namespace UI

[bbockelm]

In #264, we have a goal of putting together an initial web workflow for the registry for the v7.4.0 milestone. However, I don't want to wait until next month to get working on it - it's too large for a single issue.

To prepare for the v7.4.0 functionality, let's create a simple, minimally-functional UI for the registry in v7.3.0. The UI should:

1. Come up to a splash page a-la the origin.
2. List the registered origins in a table. Should be at least three columns -- the registered piece of the namespace, the related identity/owner (if any), and a link to download the JWKS.
3. List the caches in a table.

For now, let's hold off on authentication as a separate issue.

@haoming29 ☝️

[haoming29]

Also assigned to myself to provide related APIs for the requirement above.

[CannonLock]

There are existing APIs up for this that we can start on right away