Blueprints are broken when the name of a field in the schema changes

[psrpinto]

Steps to reproduce

• npm run start:firefox
• Click "Import Blog Posts"
• Navigate to the screen where one clicks around to fill the blogpost
• Select the title of the post
• In your editor, rename the title field in ./schema/subjects/blog-post.json to something other than title. Note that it's fields.title we're modifying, not the top-level title.
• In your terminal, do npm run build:schema
• In your editor, add some blank lines in a TypeScript file and save it, so that the extension reloads
• The following error is displayed

Unexpected Application Error!
field is undefined

SingleFieldEditor@moz-extension://1c528d92-66fa-4917-afc2-4888dbede0f5/app.js:375190:1212

[ashfame]

Documenting what we discussed: This is probably not a consideration if we can just make up a rule of not changing the field once defined. And if ever need to, wiping out the import and starting fresh could be a possible route to move forward.

I personally don't think it warrants data migration as possible handling because of changed fields.

[psrpinto]

I believe this one might be fixed, but testing it requires first addressing #174. I'll test this once a fix for #174 lands.