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.
Stop updating the fav_string attribute on posts. The column still exists
on the table, but is no longer used or updated.
Like the pool_string in 7d503f08, the fav_string was used in the past to
facilitate `fav:X` searches. Posts had a hidden fav_string column that
contained a list of every user who favorited the post. These were
treated like fake hidden tags on the post so that a search for `fav:X`
was treated like a tag search.
The fav_string attribute has been unused for search purposes for a while
now. It was only kept because of technicalities that required
departitioning the favorites table first (340e1008e) before it could be
removed. Basically, removing favorites with `@favorite.destroy` was
slow because Rails always deletes object by ID, but we didn't have an
index on favorites.id, and we couldn't easily add one until the
favorites table was departitioned.
Fixes#4652. See https://github.com/danbooru/danbooru/issues/4652#issuecomment-754993802
for more discussion of issues caused by the fav_string (in short: write
amplification, post table bloat, and favorite inconsistency problems).
Change post votes to work the same way as comment votes:
* Make the upvote arrow blue if you've upvoted the post, or grey if you
haven't. Likewise for the downvote arrow.
* Make it so you can click the upvote or downvote arrows to undo the vote.
* Don't show any notices when you vote on a post.
Also fix it so that votes work the same way on the posts page, the
comments page, and in the modqueue. Before it wasn't possible to undo
votes on the comments page or in the modqueue.
On the posts show page, in the favorites list, show favorites according
to the order they were added to the favorites table, rather than the
order they were added to the posts's fav_string.
On most posts these should be the same, but on old posts they may be
slightly different. The IDs of the first few hundred thousand favorites
don't appear to be in chronological order. Probably the original
favorite IDs were lost and recreated by a database move at some point in
Danbooru's history. The fav_string is also inconsistent with the
favorites table in some places (one contains favorites that aren't
contained by the other), which also throws off the order.
Partially addresses #4562 by eliminating one place where we depended on
the fav_string.
When favoriting/unfavoriting a post, it would also be upvoted/"un"voted
but the upvote/downvote/undo vote buttons would stay shown/hidden as
they were before unlike when pressing those buttons.
For example when you want change from favorite to merely upvoted you
would have to reload the page in between.
