SPIKE: investigate upgrading loan rules editor

Current status: We're on react-codemirror2 ^1.0.0. There are some react v16.0 compatibility warnings during the build with 1.0.0 but in the browser everything runs OK.

The current react-codemirror2 release is 5.1.0. Upgrading appears to be a straightforward process with some minor changes to how things are imported. The platform-core#snapshot test loan_renewal passes successfully. Testing manually, I had saw some issues with focus and cursor placement, e.g. clicking an item on a menu works but using the arrow keys moves the cursor within the editor although it appears to change the selection on the menu.

Summarizing, upgrading to a current release resolves the build compatibility warnings we currently see and editing appears to work correctly, although it is more finicky than in previous releases. There do no appear to be any changes to how the JSON data is interpreted or rendered. We should still plan the upgrade as we are years out of date, but this work need not be a prerequisite to changes in how we use the editor, e.g. . It may be pursued later or in parallel.