Add alums to site

[gemfarmer]

Extension of #1984

Expected Behavior

Adding alumni: true to any markdown file in the _authors directory adds some sort of flag to the authors page

Actual Behavior

This doesn't happen!

Steps to reproduce the behavior

• Determine who is an alumni in our list of authors
• Add alumni: true to their md files

This is issue is done when:

• All alumnis have an associated alumni attribute
• It flags alumni on the site
• All authors listed in _data/authors.yml have an archive page properly marked with alumni status (Ideally, we can delete that file, too.)

@coreycaitlin @gboone @awfrancisco

[gboone]

Added a checkbox to the acceptance criteria.

[coreycaitlin]

See #2021 and #2018 for continuing work.