Add functionality for formset of fieldsets and add post type Initiative

This commit adds the ability to define a formset of fieldsets, and adds the Initiative post type and meta boxes. Documentation for the formset of fieldsets will be more explicitly outlined in the README which will be in a future commit, but I will give an overview. 

1. Basically, the ability to define a fieldset is to define multiple, possibly heterogenous, fields and group them together (as outlined in the README). Formsets (being used by links which is subject to change) are another level of abstraction. It would go Field -> Fieldset -> Formset. In this, we can define a fieldset and then replicate it by just defining a few parameters. Read more in the README when it is updated. 

2. The Initiative post type is nested under an Office to which it is related.

**Warning!**
There is a link in the meta-box when editing that says 'Remove Subinitiative'. DO NOT CLICK THAT. I won't actually hurt anything but the functionality is broken right now. Fixing that in a future commit. You can still and add data, it's just the remove link that doesn't work.
Where is `$slug` being set?