Added Fanfiction.net-specific subclass for StoryOverview (still empty)

First in a line of steps to first make StoryOverview site-agnostic and then add AO3.
Added unit tests; right now mostly for the file template.
Added support for conditionals to File Template.

This allows optional lines and will be necessary in the future (and possibly help with FF.net already).
Added tests for story overview

That should make it easier to tear it all apart with the subclasses. Also, I found a bug through this already, so that's nice.
Optional items are now included conditionally
Merge

Merged changes together.