Renamed contentServerUrl in LibraryDumper to contentAccessUrl to avoid
confusion with contentServerUrl concept at Server/InternalServer level
(roughly, contentAccessUrl = contentServerUrl + "/content").
Running kiwix-serve without --catalogOnly and
--urlRootLocation resulted in non-clickable book tiles since
empty root was confused with empty contentServerURL which controlled
whether book preview links should be activated.
This commit fixes that bug and adds respective unit-tests.
It turned out that ContentResponse::m_root is no longer used.
At this point, the root parameter is dropped only from the 3-ary variant
of ContentResponse::build(), so that its all call sites are
automatically discovered by the compiler (and updated manually).
Including the other (4-ary) variant of ContentResponse::build() in this
change might result in the semantic change of expressions like
`ContentResponse::build(x, y, z)` and failure to update them.
We want to be sure that `Library` actually exists when we use it.
While it is not a silver bullet (user can still create a shared_ptr on
a raw pointer), making the `Server` keep `shared_ptr` on the library
help us a lot here.
