Interest Group/Audience pulldown in session search

[kenirwin]

This PR does a bunch of things related to audience/interest groups

• adds an audience pulldown selector to the session search page (third pulldown)
  • this one works differently than the others on the backend, relying on an external data source compiled by jekyll and delivered at /igs.json
  • should function the same as the other pulldowns from the user perspective, and the others should continue to work as expected
• in the poster data files, the audience property has been switch over from a plaintext string (e.g. IIG, AIG, PROMIG) to structured data in an array (e.g. ['IIG','AIG','PROMIG'] so that we can also...
• show the interest groups in the session-search modal results -- previously, keywords and audience were not shown in those results (e.g.: do a session search for session type=poster and then click on one of the results for the full description -- in the live site, you won't see keywords or audience listed in the session description; in this PR/preview you should see the audience and keywords listed.
• finally: adds icons to two buttons:
  • on the schedule grid page, adds a magnifying glass to the "switch to session search" button
  • on the session search page, adds a grid icon to the "switch to schedule" button
• closes #2

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[dwgreen1]

@kenirwin The only thing I noticed was that the drop-down menus for the session search seem to work independently (i.e. they cannot be AND together), but this behavior isn't new. I checked the 2020 version of the site and it behaves the same there, so I am going to approve the PR.