Implement click and drag (single action) multi-line comments

What does this MR do?

Adds the ability to "drag to select" which lines will be commented upon.

Related to: #280608 (closed)

Testing locally

To test this MR locally both unified_diff_components and drag_comment_selection must be enabled.

In a terminal window run rails console then:

Feature.enable(:unified_diff_components, Project.find_by_full_path("h5bp/html5-boilerplate"))
Feature.enable(:drag_comment_selection, Project.find_by_full_path("h5bp/html5-boilerplate"))

Substitute h5bp/html5-boilerplate with whatever project you will be testing on.

Screenshots (strongly suggested)

Screen_Recording_2020-12-11_at_4.41.49_PM

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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