Index organization/person/post related to membership

Override the toElasticsearch method so that we can index a memberships
related models, which in turn allows the user to query against the
related objects through the search API.
Index coalition-style membership relations

This allows you to query against the 'member' property where an
organization is a member of another organization in elasticsearch.
Add reIndex method to models

This allows us to call the 'reIndex' method from places other than the
model's post save callback.
Reindex membership after related objects change

When people, organizations or posts change, any memberships that they
are related to will also need to be reindexed so they have the latest
data.