Fix direct timeline pagination in the WebUI (#10126)

Fix direct timeline pagination in the WebUI (#10126)

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)