Inline codequality [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

For #2526 (closed)

This MR adds icons in the gutter of lines in the MR diff if the change made to the line has introduced a new code quality degradation. Hovering over the icon displays the description of the degradation in a tooltip. The data comes from an endpoint (!52843 (merged), !59421 (merged)) that returns the new code quality issues/degradations introduced by the MR. Since degradations are already sorted by severity (high to low), from the given array of degradations on a line, we choose the first one in the array so that we display the most severe degradation per line.

Testing instructions:

• This is a GitLab Ultimate feature, defined as inline_codequality in license.rb so you must have the appropriate license loaded to see the feature
• This feature is behind the feature flag :codequality_mr_diff_annotations - echo "Feature.enable(:codequality_mr_diff_annotations)" | rails c to enable it
• The default code quality template is designed for docker-in-docker, but I would recommend setting up a specific local runner using these instructions if you want to fully test this in your gdk

Task checklist

• rebase this once !56710 (merged) is merged (remove the shared commits that enable the POC for now)
• rebase/update this once !58833 (merged) is merged (which moves the store code into ee/ and changes the structure a bit)
• peel back some of this code and put it into ee/ files instead
• figure out if/when we should show these icons in the left panel of the side-by-side view
• add bits to the code theme scss to make the background of the new space match its surroundings (like inline coverage did)
• retake screenshots now that the theme scss's have been updated
• should we display the space when the feature is not available?
• write tests
• write a better description
• rebase now that related MRs are merged

Screenshots

view	before (or without code quality data)	after (with code quality data)	in each code theme
inline			Screen_Recording_2021-05-07_at_14.09.29
side-by-side			Screen_Recording_2021-05-07_at_14.10.25

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 feature is behind the feature flag :codequality_mr_diff.
• 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