Implement the timeline marker endpoints

[akirk]

For syncing the timeline position between apps, we'd need these endpoints implemented:

• [ ] GET /api/v1/markers (implemented as empty) Get saved timeline positions
• [ ] POST /api/v1/markers Save your position in a timeline

[akirk]

For https://github.com/akirk/friends/issues/399 it would be interesting to save this in a non-namespaced user option so that both Friends and EMA can set it.