This commit adds a new endpoint `/v1/events` which provides a list of
all timeline events. It is able to be filtered by story_id, worklist_id,
and board_id.
The existing `/v1/stories/:id/events` endpoint is preserved, but similar
endpoints for worklists and boards don't exist in favour of using something
like `/v1/events?worklist_id=360`.
Change-Id: Ie13f9d71d9a0b736e4184dd36cc1a6ac26a5f109
d7efbd8cd4