Commenting on commits associated with merge request additional styling

Follow up of https://gitlab.com/gitlab-org/gitlab-ce/issues/31847

Merge request commits tab

• Update the icon

Merge request changes tab

Message bar

• The background color is #FFFAF4.
• It is moved to below diff navigation.

• When the box and warning message stack up, it doesn't have a good visual hierarchy since those 2 elements are both prominent. (One is in a box with the gray background and another one has the orange background)

• Icon: warning on gitlab-svgs

Commit message box

• The background of detail commit message is #F2F2F2 and make it look like uneditable.

Commits view

A discussion on commit outside of merge request (Discussion tab)

• Copy: User @username started a discussion on commit 1a2b3c4d outside of this merge request a hour ago
• Resolve discussion button is disabled.
• When the mouse is over the Resolve discussion icon / button, the tooltip with "Commit comments made outside the context of this merge request cannot be resolved." shows up.

Button	Icon