move HTML <head> partial to new partials/ folder
fix errors due to submodule moving

I'm pretty sure I already did that - must have been lost somehow,
possibly during merging?
move header and footer to partials/ folder
correct github link
move internal PHP to new subfolders

Code that is used via `require()` or `require_once()` goes
in the `util/` subfolder, whereas code that is exposed as
internally used URL goes in the `internal/` subfolder.

Also, delete an unused util file.
adjust paths to file movings

The paths passed to require() and require_once() the moved
files have been updated. Also, such calls inside the moved
files needed a fix. And last but not least, the references
to the internal URLs have been corrected by this commit.

NOTE:
In the moved files, some require_once() paths needed to be
prefixed with the directory path containing them, because
when required, the working directory is different. And for
PHP < 5.2 support, __DIR__ could not be used.