assets/diff-utils: Use homemade missing new line icon instead of GH one
Create a missing new line at end of file icon by combining two Material Design ones instead of reusing a SVG copy of the old GitHub one.
This removes a copyright infringement while having a better looking icon.
For the record, I also noticed flaws in the way I implemented the display of that icon in diff views and the feature also misses a cypress test. I will handle that in another diff.
Migrated from D5959 (view on Phabricator)