Clean up helpers code

[bantic]

This follows on #69 to clean it up a little bit.

• Add "import { helper } from '@ember/component/helper" and an export to all the helpers in fixtures inputs
• Update the corresponding expectation in the fixtures outputs
• Add common-tags as a devDep to allow using stripIndents
• Extract the helper codemod code to a separate file
• Modify lib/models/file-info.js to apply the helper codemod to the output of the importDeclarationsTransform

[rwjblue]

Thank you!!!