[Core API] Draft and conversation sharing

Added support for sharing drafts and adding/removing teammates as followers of a conversation.

Added

New property draft_mode on the Message data model. Its value can be either null (for a non-draft message), "private" (for a draft visible to its author only) or "shared" (for a draft visible to all teammates with access to the conversation
New optional body field mode when creating a draft to decide which mode to create the draft into. It supports two values: "private" (default) and "shared".
New optional body field mode when editing a draft. It only support the value "shared" to make a private draft shared.
New endpoints POST /conversations/:conversation_id/followers and DELETE /conversations/:conversation_id/followers to control the teammates following a conversation.

The property is_draft of the Message data model is now deprecated in favor of the new property draft_mode.