Explain why we have a default haystack

[mccalluc]

Fix #1822? I've looked at the git history a little bit, without getting a clear sense of how we got to the present state.

[mccalluc]

  File "/home/travis/virtualenv/python2.7.13/lib/python2.7/site-packages/haystack/__init__.py", line 41, in <module>
    raise ImproperlyConfigured("The default alias '%s' must be included in the HAYSTACK_CONNECTIONS setting." % DEFAULT_ALIAS)
django.core.exceptions.ImproperlyConfigured: The default alias 'default' must be included in the HAYSTACK_CONNECTIONS setting.

http://django-haystack.readthedocs.io/en/v2.4.1/multiple_index.html

Haystack requires a "default", but there's no benefit to making it either of our two. I'll add that section back in, but explain why.

[codecov-io]

Codecov Report

Merging #1823 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1823   +/-   ##
========================================
  Coverage    41.04%   41.04%           
========================================
  Files          394      394           
  Lines        25758    25758           
  Branches      1278     1278           
========================================
  Hits         10572    10572           
  Misses       15186    15186

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