A mobile website should be sent the proper user agent (server side).

When running a command like so:

    ripple emulate --remote http://foo.com

Then navigate to the logged (local) url:

    http://localhost:PORT?enableripple=true

When enabling for that page, the current userAgent of the current
device (according to client side db) will be sent to the server and
used instead of the browser's.

Note: This does not cover some edge cases, such as having links with
hardcoded host names in them.

Note: Also cleaned up the current navigator.userAgent (client side)
emulation to be more simple (i.e. if userAgent on the current device is
falsy, use original).
The `pkg/web` build target is no longer warranted.

This build target is no longer (primarily) used, and now that there is a
hosted build target, which can function without the chrome extension
(i.e. standalone) this is no longer needed, and should be removed.

This also cleans up some hacky (complexity inducing) code (such as the
omnibar checks) that has never felt right.
There were some incorrect (relative) image paths in the main css file.

It did not cause any visible breakages in the Chrome Extension build
target, but in the hosted version (where assets are loaded from
`ripple/assets`) it shows up.