LaTeX template: Restrict institute to Beamer

[adunning]

Use the etoolbox package (same technique as #5213) to add the institute variable to \maketitle. Verified to work with article, book, report, memoir, scrartcl. Closes #4209.

It's not the prettiest thing ever: the tabular environment of \author and \and is rather inflexible. I'm wondering if the institute variable was intended only to support Beamer users (in which case it could simply be excluded from normal LaTeX output), but the manual indicates that it's supposed to work in both LaTeX and Beamer.

Example:

pandoc -o institute.pdf << EOT

---
title: Test
subtitle: More tests
author:
- Tester 1
- Tester 2
institute:
- University of Testing
- University of Diagnostics
date:
- 2019
---

EOT

[adunning]

It looks slightly better with reduced spacing:

[adunning]

In case it wasn't clear from the above, I don't necessarily think that accepting this PR is the best approach. I can see at least three ways of approaching this:

1. Use this PR to render \institute. It might be handy to someone, and would follow what the manual states – but its value is limited given that it doesn't work in any other format, and I doubt that anyone wants to work through all the writers to support it elsewhere.
2. Include the variable only in Beamer output, which supports it natively.
3. Drop the variable altogether and expect anyone who wants it to insert it to use header-includes. This would be most consistent in light of the template's exclusion of other non-standard variables, such as the \subject and \publishers commands in KOMA-Script.

The institute variable was originally added to the template in 7688f0f with the intent of using it for articles, but for a journal-specific class that isn't even available through TeX Live, and it doesn't appear that the author realized it could be inserted using header-includes.

[jgm]

institute was indeed intended only for beamer. This could be clarified in the docs.
And maybe it should be made conditional on beamer in the template?

[adunning]

That's certainly cleaner; I've added another commit for this.

[adunning]

Looks like the testing system is having a bad day; this works locally.