LaTeX/Beamer templates: Remove conditionalization around date.

This restores the old behavior, where no date was printed
if no date was specified.  If the `\date` command is removed
entirely, standard document classes will print today's date.
Closes #1321.

Partially reverts fc5229df167aa61cdbcf14ce24581b36bb0a5b90.
See #61.
LaTeX template:  fix subtitles.

The old code really requires scrartcl from koma-script.
Closes #1327.
Fixed subtitle in latex template.

See jgm/pandoc#1327.
Merged in changes from JGM's templates repo