Create custom metaboxes for project metadata

[benlk]

This ticket captures the yet-to-be-completed editor-side interfaces described in #131:

Do we want to try to finish the custom meta box for this, or is the standard "Custom Fields" editor enough for admin users? The Custom Fields editor doesn't load all registered custom fields, and so does not provide an easy way for editors to know or set these fields if they are not already set on the post

This meta box ticket covers:

• Create a metabox with fields for all custom post meta registered by this plugin
• Save action for same

[benlk]

The list of fields is captured in https://docs.google.com/spreadsheets/d/1eHTJV1OKOWfOrZAaPosovc96dfhn1A_-SdRyMWge8Ts/edit#gid=0

[MirandaEcho]

This encompasses:

• Creating first metabox and testing (4 hours)
• Copy paste & testing for additional metaboxes (3 hours)