Create attribution page

Using the metadata pulled down from the server, the bundler will call
out to parsoid to render a attribution page. The attribution page will
be added to the bundle as `attribution.html` (and optionally
`attribution.wt`) and a new metabook item will be added with type
'attribution'.

Relies on:
* https://gerrit.wikimedia.org/r/#/c/145189/
* https://gerrit.wikimedia.org/r/#/c/145192/

Change-Id: I85d578ef104d86beaf4d8d31b38794bc92c01c67