Show code quality badge on files in MR diff view

What does this MR do?

For #267612 (closed)

Show a badge in the file header of each file in the MR diff/changes tab that has code quality changes, i.e. if the MR affects the code quality of a given file, show a badge next to its filename in the diff view

Technically, this MR:

• adds inline_codequality as a licensed feature for GitLab Ultimate
• is behind the codequality_mr_diff feature flag
• outputs the code quality diff endpoint in haml to be consumed by the diffs vue app only if both the feature flag and the licensed feature are enabled/available
• modifies the diffs vue app to fetch data from this endpoint if it is provided and display badges on changed files:
  • uses the same polling procedure as inline code coverage
  • stores the result in the diffs store as codequalityDiff
  • passes along code quality diff data for each file to diff_file.vue as codequalityDiff prop (defaults to [])
  • if a diff_file has codequalityDiff.length > 0, passes along hasCodequalityChanges prop to diff_file_header.vue
  • if a diff_file_header hasCodequalityChanges, display the code quality badge component
• add the code quality badge component
  • in ee/ because this will be a GitLab Ultimate feature
  • asynchronously import the EE child component

Screenshots (strongly suggested)

Screen_Recording_2021-03-15_at_15.32.08

link-to-widget functionality has been split off from this MR and will be implemented in !57791 (merged)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this change is behind a feature flag.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done