Fix sites handling

[johnbaldwin]

New addition to filter sites is breaking in devsite multisite mode. There may be community installation issues too

https://github.com/appsembler/figures/blob/master/figures/sites.py#L312

  File "/Users/jbaldwin/work/appsembler/repos/figures/devsite/devsite/seed.py", line 476, in seed_all
    backfill_figures_ed()
  File "/Users/jbaldwin/work/appsembler/repos/figures/devsite/devsite/seed.py", line 415, in backfill_figures_ed
    for site in get_sites():
  File "/Users/jbaldwin/work/appsembler/repos/figures/figures/sites.py", line 312, in get_sites
    sites_backend_path = settings.ENV_TOKENS['FIGURES'].get('SITES_BACKEND')
KeyError: 'FIGURES'