Enhance display for repeated notices

When receiving repeated notice, current Mustard Mod version displays it as "old style" RT, begining with "RT @username". Therefore, text is often more than 140 characters long and got truncated. Since we often have links at the end of notices, we're unable to open them.

Theses 3 commits change the way repeated notices are handled:
- extract & store the original notice in DB, keeping track of repeated `status_id`, repeated `created_at` timestamp (to keep merged timeline working) as well as repeater's `screen_name` (to display who repeated notice).
- Add new layout field to display who repeated the notice. Currently displayed like "RT by screen_name".
- Adapt timeline loader to use `repeated_id` and not original `status_id` (so that old notice which were recently repeated won't prevent to load more notices)