[Security Solution] [Timelines] Copying a timeline does not also copy a timeline's notes

[kqualters-elastic]

Describe the bug:
When duplicating a timeline, notes are not also duplicated, but the code in x-pack/plugins/security_solution/server/lib/timeline/saved_object/timelines/index.ts suggests they should be.

Kibana/Elasticsearch Stack version:
Unclear how long this has been an issue.

Steps to reproduce:

1. Create and save a timeline with notes, either on an event or only on the timeline.
2. Duplicate that timeline.
3. See that this new timeline has 0 notes.

Current behavior:

Expected behavior:
A copy of each note is created as well.

[elasticmachine]

Pinging @elastic/security-threat-hunting-investigations (Team:Threat Hunting:Investigations)