Fix for "more" link on mobile view

Changes in "more"-button for mobile view.
Previously the "more"-button was displayed if there were any posts displayed on the page.

Now the "more"-button is only shown if the current page is full (=15 posts), so there is a good chance there might be more posts available.

Also I added a message to mark the end of posts available for the stream.
This will serve two purposes:
- If you have a stream with exactly 15 * x posts in it, your second to last page will have a "more"-button and the next page will be empty. Now there will be a message informing you why its empty, signaling the user that diaspora is not broken and it's the right result for the users action.
- If the user expects the more-button at the end of a page, but the page has < 15 posts, the "no more posts" message is displayed to signal that there is no "more"-button for a reason.

This will fix #5003.
I think it makes sense to do `- elsif params[:max_time].present?` here and to add an `else` branch saying something like `There are no posts yet`, similar to the empty stream message on the desktop site.