Fix some issues with v2

[rafaucau]

resolves #101

• Improved performance by eliminating unnecessary database queries
• Fixed error when discussion author has deleted account

[rafaucau]

@jaspervriends what is minimum supported PHP version by this extension?

[rafaucau]

This causes a huge performance issue when the discussion contains thousands of posts:

flarum-seo/src/Page/DiscussionBestAnswerPage.php

Lines 173 to 174 in da3cb93

	$posts = $discussion->posts()
	->where('number', '>', '1')->get();

What would be the preferred approach for handling this - limiting the number of loaded posts or skipping this functionality entirely for discussions with a high post count? I can submit a separate PR once we determine the best solution. Meanwhile, this PR can be merged as it improves other aspects.