Restructure project to support modularity/flexibility

[jthomale]

Right now the structure of the project isn't necessarily very flexible, especially when considering use/deployment at other institutions.

• The entire /solr directory maybe could be removed, or moved out of the project, except the certain configuration files (schemas, etc.). Especially if we moved to a pure pymarc implementation of MARC exports (and got rid of the dependence on SolrMarc).
• The base API implementation: what if an institution wants to completely customize it, or throw it out altogether and start fresh? In the api app, it's mostly Django REST Framework views and serializers, which are pretty tied to the API data structure. But, there are also some base class definitions and other tools that are useful for building API data structures. Maybe those need to be separated.
• Deployment is complicated. Need to simplify.