Migrate all weight system notes ending in a comma to the new format
This should be like https://gitlab.com/gitlab-org/gitlab-ee/blob/2a0ad260f5f45b777068ce9a292322491e25a1d9/ee/db/migrate/20180530201303_remove_commas_from_weight_note.rb, but run as a background migration.
Some data from a previous test is at https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5854#note_79865544 and https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5854/diffs#note_80045735