The `hasMore` property of timelines in redux store was set whenever an API
request returned only one page of results, *even* if the query only requested
newer conversations (using `since_id`), causing `hasMore` to be incorrectly set to
false whenever fetching new toots in the direct timeline, which happens each time
the direct message column is opened.
(Basically #9516 for direct messages)
* Add conversations API
* Add web UI for conversations
* Add test for conversations API
* Add tests for ConversationAccount
* Improve web UI
* Rename ConversationAccount to AccountConversation
* Remove conversations on block and mute
* Change last_status_id to be a denormalization of status_ids
* Add optimistic locking
Eugen Rochko