Persist portfolios, part 1

[hancush]

Overview

This PR:

• Sets up Django REST Framework and adds a view set for Portfolios
• Updates the CIP planner:
  • Passes the last updated portfolio belonging to the authenticated user, if one exists (#89)
  • Adds a button to conditionally update the selected portfolio (#87) or create a new one (and subsequently update it, after it is created) (#86)

There will be merge conflicts as a result of some file renaming and minor changes in #93, but we'll deal with that later.

TODO

• The save UI needs to be improved. Either prevent navigating away from the page with unsaved changes, or force save before going ahead with navigation ("Be able to toggle between portfolio and a project page without losing projects in unsaved portfolio.")
• Add a way to select from any of the user's portfolios (#88)

Testing Instructions

• View the CIP planner
• On first load, confirm you see an empty portfolio
• Add some phases and confirm the totals, etc., update as normal
• Click save and confirm the button becomes disabled
• Refresh the page and confirm the CIP planner reloads rehydated with your new portfolio. Also confirm the remaining projects list does not contain projects already in the portfolio

[hancush]

@smcalilly I think we can keep the data table views the same for now, since the BaseDataTableView is doing some extra work for us that I don't want to reimplement in DRF.

[hancush]

@smcalilly Once this secures your approval, can I merge into master, would you prefer to wait to merge / deploy to staging until the full UI is done? (I have another changeset chilling on a separate branch.)