Add source references to rendered shtml

[kohlhase]

The functionality (see #39) is currently broken (e.g. on the functions.en the button does nothing).

[tkw1536]

The actual bug isn't that the button is broken, it's that the frontend thinks there is a source reference while there isn't any.

[tkw1536]

(Report issue buttons are based on source references)

[tkw1536]

There are now source references, although they point to the xhtml which obviously doesn't exist in the git repository. Might have to work on this further in the future.
Reporting an issue works though.

[kohlhase]

much better.