Fix eslint issues in the file hierarchy table code

[robyngit]

In preparation for extending the file hierarchy table to be used in the editor, we should start with some cleanup of the existing code. This will make it easier to add new functionality and ensure that the code is maintainable. To start, we will address some eslint issues in files that are related to the file hierarchy table.

Fix all eslint issues in the following files:

• [x] src/js/views/DataItemView.js
• [x] src/js/views/DataPackageView.js
• [x] src/js/views/DownloadButtonView.js
• [x] src/js/views/MetadataView.js
• [ ] src/js/collections/DataPackage.js
• [x] src/js/models/PackageModel.js

Then:

• [x] Ensure formatting is still correct with these changes: npm run format

[robyngit]

Note that I'm going to fix the remaining MetadataView linting errors as part of #1758