Github Importer - Import pull request reviews

What does this MR do?

Github has the concept of pull request reviews, where a user can add a comment with the type: "APPROVED", "CHANGES_REQUESTED" or "COMMENTED".

This commit introduces the ability to import this type of comments from Github.

For the "APPROVED" type, if the author of the comment can be mapped to a Gitlab user, an merge request approval is created.

For all the types a note is added to the merge request. If the author of the review cannot be mapped to a Gitlab user a complementary text is added to the original review body informing the original author of the review.

Screenshots (strongly suggested)

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team