9821 Major FS refactoring

Building on the file-security branch (gh-556), this refactoring contains the first changes for the 4.5 database model as well as an asynchronous API for performing imports. Assuming testing goes well with this, I'll close 556.

See the ["FS Specification Draft"](https://docs.google.com/document/d/15zpLHsXhvSbCa5hLIn9Mwj3spCq-2wvN9QXLZCao3ZU/edit#) gdoc for an explanation of how this branch works.


/cc for model changes: @qidane, @melissalinkert, @rleigh-dundee
/cc for API changes: @knabar, @jburel, @chris-allan