Fix N plus one queries for new review email

What does this MR do and why?

In #355027 (closed), we found several N+1 and N+1 cached queries executed for new_review_email.

N+1 Queries

sql	count	line
SELECT "notes".* FROM "notes" WHERE "notes"."noteable_id" = [id] AND "notes"."noteable_type" = 'MergeRequest' AND "notes"."discussion_id" = [filtered] ORDER BY "notes"."created_at" ASC, "notes"."id" ASC	65	/app/models/note.rb:178:in `find_discussion'
SELECT "note_diff_files".* FROM "note_diff_files" WHERE "note_diff_files"."diff_note_id" = [id] LIMIT 1 /*application:console,db_config_name:main,line:/app/models/diff_note.rb:144:in `should_create_diff_file?'	65	/app/models/diff_note.rb:144:should_create_diff_file?

N+1 cached queries

sql	count	line
SELECT "users".* FROM "users" WHERE "users"."id" = [user_id] LIMIT 1"	85	"app/mailers/emails/reviews.rb:16:in `review_thread_options'"
SELECT "routes".* FROM "routes" WHERE "routes"."source_id" = 1 AND "routes"."source_type" = 'Project' LIMIT 1	85	"app/models/concerns/routable.rb:118:in `full_path'"
"SELECT "merge_requests".* FROM "merge_requests" WHERE "merge_requests"."id" = 27 LIMIT 1"	64	"app/models/note.rb:337:in `noteable'"
SELECT "projects"."id", "projects"."name", "projects"."path", "projects"."description", "projects"."created_at", "projects"."updated_at", "projects"."creator_id", "projects"."namespace_id", "projects"."last_activity_at", "projects"."import_url", "projects"."visibility_level", "projects"."archived", "projects"."avatar", "projects"."merge_requests_template", "projects"."star_count", "projects"."merge_requests_rebase_enabled", "projects"."import_type", "projects"."import_source", "projects"."approvals_before_merge", "projects"."reset_approvals_on_push", "projects"."merge_requests_ff_only_enabled", "projects"."issues_template", "projects"."mirror", "projects"."mirror_user_id", "projects"."shared_runners_enabled", "projects"."runners_token", "projects"."build_coverage_regex", "projects"."build_allow_git_fetch", "projects"."build_timeout", "projects"."mirror_trigger_builds", "projects"."pending_delete", "projects"."public_builds", "projects"."last_repository_check_failed", "projects"."last_repository_check_at", "projects"."only_allow_merge_if_pipeline_succeeds", "projects"."has_external_issue_tracker", "projects"."repository_storage", "projects"."repository_read_only", "projects"."request_access_enabled", "projects"."has_external_wiki", "projects"."ci_config_path", "projects"."lfs_enabled", "projects"."description_html", "projects"."only_allow_merge_if_all_discussions_are_resolved", "projects"."repository_size_limit", "projects"."printing_merge_request_link_enabled", "projects"."auto_cancel_pending_pipelines", "projects"."service_desk_enabled", "projects"."cached_markdown_version", "projects"."delete_error", "projects"."last_repository_updated_at", "projects"."disable_overriding_approvers_per_merge_request", "projects"."storage_version", "projects"."resolve_outdated_diff_discussions", "projects"."remote_mirror_available_overridden", "projects"."only_mirror_protected_branches", "projects"."pull_mirror_available_overridden", "projects"."jobs_cache_index", "projects"."external_authorization_classification_label", "projects"."mirror_overwrites_diverged_branches", "projects"."pages_https_only", "projects"."external_webhook_token", "projects"."packages_enabled", "projects"."merge_requests_author_approval", "projects"."pool_repository_id", "projects"."runners_token_encrypted", "projects"."bfg_object_map", "projects"."detected_repository_languages", "projects"."merge_requests_disable_committers_approval", "projects"."require_password_to_approve", "projects"."emails_disabled", "projects"."max_pages_size", "projects"."max_artifacts_size", "projects"."remove_source_branch_after_merge", "projects"."marked_for_deletion_at", "projects"."marked_for_deletion_by_user_id", "projects"."autoclose_referenced_issues", "projects"."suggestion_commit_message", "projects"."project_namespace_id", "projects"."hidden" FROM "projects" WHERE "projects"."id" = 1 LIMIT 1	64	"app/models/note_diff_file.rb:10:in `project'"

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Related to #355027 (closed)