Move the diff file conflict error alert to be above the diff file header

What does this MR do and why?

For #456497 (closed)

Moves the conflict warning for a file to be above the file title/header.
This arrangement more closely aligns with error positioning of other UI elements (above the beginning of the otherwise normal content).

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How to set up and validate locally

1. Have an MR with a conflicted file
   1. It will be useful to also have other files in the MR, to see how the conflicted file looks among others
2. Visit the MR's Changes tab