Allow application of settings via GET request.

This allows us to request that a given collection be printed on
letter-sized paper (for instance) using a URL containing the query
string:

  ?writer=rdf2latex&papersize=letter

We can similarly set the number of columns, the presence of a table
of contents, and all of the other options available via book creator.

Change-Id: I0587a97a869a3b6082401338f2481341710ec0cb
Add a top-level `settings` key for compatibility with the old mw-serve API.

This was removed in 677f1a1907455921efabb0adcc762727d9539843 but we're
adding it back for backwards compatibility.

Change-Id: I9243105b030e18445085fed27fce4d2eb1ad6203
Merge "Allow application of settings via GET request."
Merge "Add a top-level `settings` key for compatibility with the old mw-serve API."