Blob view: add storybook tests

Created by: olafurpg

While investigating why our CI didn't catch the regression that caused INC-140, I found out that are no React storybook tests for the BlobPage component. We have storybook tests for search results, highlighted results in the reference panel, but no tests for the blob itself. We should add storybook tests ASAP to prevent similar regressions from happening in the future. It would also be helpful to have storybook tests for the new CodeMirror blob view so that we can more easily review changes related to it's UI (search component, highlighting, gutters, etc).