Prevent “This merge request contains merge conflicts” from being shown after resolution because of slow background workers

Some SCSS Foo in action! 💥

This is more efficient for large files than performing a regex match on
every single line.

This reverts commit 2536fea8db0967a817615eb3e951eb6ba22f8ede.

An MR can only be resolved in the UI if:
- It has conflicts.
- It has valid diff_refs (in other words, it supports new diff notes).
- It has no conflicts with one side missing.
- It has no conflicts in binary files.
- It has no conflicts in files too large to display.
- It has no conflicts containing invalid conflict markers.

- Add match line header to expected result for `File#sections`.
- Lowercase CSS colours.
- Remove unused `diff_refs` keyword argument.
- Rename `parent` -> `parent_file`, to be more explicit.
- Skip an iteration when highlighting.