OutputPage: Make use of WrappedStringList in headElement()

OutputPage: Make use of WrappedStringList in headElement()

Right now, getInlineHeadScripts(), buildCssLinks() and getExternalHeadScripts()
all return WrappedString::join(). But because they don't know about each other
and because they need to return strings (not arrays), headElement() has no way
of merging them.

WrappedStringList allows this array to be kept, whilst still being backward-compatible
with code that calls these methods and assumig a string (since it will lazy-join
the array if the object is treated like a string).

To be used by I8b6c6a10d965e7396. Output is not changed in this commit.
Merely refactoring.

Change-Id: Iae08345473bd93cc0948d51b62c48aeb1ea460a3