Improve UI for creating portfolios

[hancush]

Overview

This PR:

• Adds an explicit button for editing portfolio name
• Prompts the user about unsaved changes if they try to navigate away from the current portfolio
• Adds a portfolio picker to switch portfolios

Notes

TODO:

• [x] Default to editable name if no portfolios exist
• [x] Update the portfolio picker when a new portfolio is created.

Testing Instructions

• Navigate to the review app at https://ccfp-asset-d-hcg-portfo-ratya9.herokuapp.com/cip-planner/ and log in using the test user creds in LastPass
• Confirm a portfolio with one phase is rendered by default
• Try removing the phase, then refreshing the page. Confirm you see a dialog confirming you want to navigate away. Also try clicking links to other pages, etc., and confirm the dialog again appears.
• Create a new portfolio. Try adding/removing phases, saving, etc., and confirm the page state reflects your changes (or lack thereof).
• Refresh the page and confirm the last portfolio you edited is active. (Need to add functionality to update the select with a new option when a portfolio is created, will do Monday.)
• Select the other portfolio from the dropdown and confirm it populates the page correctly.
• Try to break the page in other ways I haven't thought of and let me know what doesn't work. :-)

[hancush]

Excellent UI suggestions, @smcalilly! It looks a lot better. This is ready for re-review.