Wrap the whole diff file when a conflict badge is present

The following discussion from !149006 (merged) should be addressed:

• @slashmanov started a discussion: (+2 comments)

  It feels kinda weird that the outline doesn't wrap the error message, like these things are separate from each other, even though they are both parts of a single file. @annabeldunstone do you think we need to change this?

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Design proposal

• Apply border to entire diff file container
• Position alert at the top of the diff file to align with how MR widgets work