services/identity: Give Identity Service a makeover.

The identity server now both uses v.io's CSS and has some of its own.

The identity service should be started with --assetsprefix flag that has the
host that serves images, css, etc that it needs to look pretty.
The assets in pracitve will be hosted from v.io.

This change also changes relative links starting with "/" to "/auth" to
allow the links to work through a proxy. This means the identity server
serves all its stuff at host:port/auth/...

Here are some screenshots to make it easier to review :)
Home Page: https://screenshot.googleplex.com/rEDCeBLB5j.png
Add Caveats Page: https://screenshot.googleplex.com/AVMDyGzLNJ
View Blessings Page: https://screenshot.googleplex.com/DNtUFNOOLT

MultiPart: 1/3
Change-Id: I93029095b84f28d5fad1f80b6b92ea796daf106c
14 files changed