LaTeX template: improve treatment of CSL entry-spacing.

Previously with the default template settings (`indent` variable
not set), we would get interparagraph spaces separating bib
entries even with `entry-spacing="0"`.  On the other hand,
setting `entry-spacing="2"` gave ridiculously large spacing.
This change makes the spacing caused by `entry-spacing` a multiple
of `\parskip` by default, which gives aesthetically reasonable
output.  Those who want a larger or smaller unit (e.g. because
they use `indent` which sets `\parskip` to 0) may
`\setlength{\cslentryspacingunit}{10pt}` in header-includes
to override the defaults.

Closes #7296.

data/templates/default.latex

@@ -378,6 +378,8 @@ $if(csl-refs)$
 \setlength{\cslhangindent}{1.5em}
 \newlength{\csllabelwidth}
 \setlength{\csllabelwidth}{3em}
+\newlength{\cslentryspacingunit} % times entry-spacing
+\setlength{\cslentryspacingunit}{\parskip}
 \newenvironment{CSLReferences}[2] % #1 hanging-ident, #2 entry spacing
  {% don't indent paragraphs
   \setlength{\parindent}{0pt}
@@ -387,9 +389,7 @@ $if(csl-refs)$
   \def\par{\hangindent=\cslhangindent\oldpar}
   \fi
   % set entry spacing
-  \ifnum #2 > 0
-  \setlength{\parskip}{#2\baselineskip}
-  \fi
+  \setlength{\parskip}{#2\cslentryspacingunit}
  }%
  {}
 \usepackage{calc}