Improve the Commit box on the Merge Request Changs tab when browsing per commit

What does this MR do?

Fixes:

• Broken user avatar (should be besides the commit info) (#254706 (closed))
• Makes the Commit expanded description not bound to the container (it can grow full width) (#240888 (closed))
• Stacks the commit actions on narrower screen widths
• Note: It changes the markup order (uses flex to reverse it to the proper visual order): this might affect user navigation, tabbing through the Actions before the Commit link

Screenshots

Before	This MR
Screen_Recording_2020-09-28_at_20.46.08_480	Kapture_2020-09-28_at_20.29.51

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

Closes #254706 (closed)

Closes #240888 (closed)