Extend URL variable expansion syntax and add source links to the contents page

Like the wiki link on the contents and index page, add a source code link too.
Extend the wiki & source URL variable expansion syntax.

The original syntax was:
%F for the source file name (the .hs version only, not the .lhs or .hs.pp one)
%M for the module name (with '.' replaced by '/')

The new syntax is:
%F or %{FILE} for the original source file name
%M or %{MODULE} for the module name (no replacements)
%N or %{NAME} for the function/type export name
%K or %{KIND} for a type/value flag "t" or "v"

with these extensions:
%{MODULE/./c} to replace the '.' module seperator with any other char c

%{VAR|some text with the % char in it} which means if the VAR is not in use in
this URL context then "" else replace the given text with the '%' char
replaced by the string value of the VAR. This extension allows us to construct
URLs wit optional parts, since the module/file name is not available for the
URL in the contents/index pages and the value/type name is not available for
the URL at the top level of each module.