Partially addresses #47424.

Improve performance of DiffDiscussion#truncated_diff_lines and DiffNote#diff_line by removing expensive diff position calculation and comparison

This cop has been renamed to ActiveRecordSerialize to match the way
"ActiveRecord" is usually written.

This Cop blacklists the use of ActiveRecord's "serialize" method, except
for cases where we already use this.

So we have raw_diffs too

Extract parts of LegacyDiffNote into DiffOnNote concern and move part of responsibility to other classes

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb