Changes:
* Make it so you can click or hover over a post's favorite count to see
the list of public favorites.
* Remove the "Show »" button next to the favorite count.
* Make the favorites list visible to all users. Before favorites were
only visible to Gold users.
* Make the /favorites page show the list of all public favorites,
instead of redirecting to the current user's favorites.
* Add /posts/:id/favorites endpoint.
* Add /users/:id/favorites endpoint.
This is for several reasons:
* To make viewing favorites work the same way as viewing upvotes.
* To make posts load faster for Gold users. Before, we loaded all the
favorites when viewing a post, even when the user didn't look at them.
This made pageloads slower for posts that had hundreds or thousands of
favorites. Now we only load the favlist if the user hovers over the favcount.
* To make the favorite list visible to all users. Before, it wasn't
visible to non-Gold users, because of the performance issue listed above.
* To make it more obvious that favorites are public by default. Before,
since regular users could only see the favcount, they may have
mistakenly believed other users couldn't see their favorites.
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