REST: Add 'web_url' link to API responses

REST: Add 'web_url' link to API responses

This provides an easy way for clients to navigate to the web view. The
URL is added to four resources: bundles, comments, cover letters and
series. We could also extend this to projects and users in the future,
but the latter would require renaming an existing property while the
latter would require a public "user" page which does not currently
exists.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Reviewed-by: Daniel Axtens <dja@axtens.net>