LaTeX template: Restrict institute to Beamer (#5219)

The `\institute` command is only standard in the Beamer class.
Use a conditional to restrict this to Beamer output rather than
output an empty command. To add this information to a LaTeX
class providing an `\institute` command, use `header-includes`.

MANUAL.txt

@@ -1370,10 +1370,6 @@ depending on the output format, but include the following:
 `subtitle`
 :   document subtitle, included in HTML, EPUB, LaTeX, ConTeXt, and Word docx
 
-`institute`
-:   author affiliations (in LaTeX and Beamer only).  Can be a
-    list, when there are multiple authors.
-
 `abstract`
 :   document summary, included in LaTeX, ConTeXt, AsciiDoc, and Word docx
 
@@ -1453,6 +1449,10 @@ including all [reveal.js configuration options].
 `logo`
 :   logo for Beamer documents
 
+`institute`
+:   author affiliations in Beamer.  Can be a
+    list when there are multiple authors.
+
 `slidy-url`
 :   base URL for Slidy documents (defaults to
     `https://www.w3.org/Talks/Tools/Slidy2`)

data/templates/default.latex

@@ -364,12 +364,11 @@ $endif$
 $if(author)$
 \author{$for(author)$$author$$sep$ \and $endfor$}
 $endif$
+\date{$date$}
+$if(beamer)$
 $if(institute)$
-\providecommand{\institute}[1]{}
 \institute{$for(institute)$$institute$$sep$ \and $endfor$}
 $endif$
-\date{$date$}
-$if(beamer)$
 $if(titlegraphic)$
 \titlegraphic{\includegraphics{$titlegraphic$}}
 $endif$