slides-preamble.tex: Add missing file

This has apparently been missing all of this time; I'm sorry!

Thanks, Amin Bandali, for letting me know.

.gitignore

@@ -1,6 +1,6 @@
 # generated
 /*.pdf
-/*.tex
+/slides.tex
 /*.blg
 /texput.log
 /*.bbl

slides-preamble.tex

@@ -0,0 +1,28 @@
+% Beamer configuration for slides.org
+%
+%  Copyright (C) 2021 Mike Gerwitz
+%
+%  This program is free software: you can redistribute it and/or modify
+%  it under the terms of the GNU General Public License as published by
+%  the Free Software Foundation, either version 3 of the License, or
+%  (at your option) any later version.
+%
+%  This program is distributed in the hope that it will be useful,
+%  but WITHOUT ANY WARRANTY; without even the implied warranty of
+%  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+%  GNU General Public License for more details.
+%
+%  You should have received a copy of the GNU General Public License
+%  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+\beamertemplatenavigationsymbolsempty
+
+\setbeamertemplate{headline}{}
+\setbeamertemplate{page number in head/foot}{}
+
+\setbeamercolor{title}{bg=white}
+
+% Centering kluge (\aftergroup needed if slide does not contain subsections
+% like notes in the TeX output, so just do both and hope for the best).
+\def\fullslidetext{\Huge\aftergroup\centering\centering}
+\def\fullsubtext{\medskip\Large}