policies: remove current request from context.

This refactors Pundit policies to only rely on the current user, not on
the current user and the current HTTP request. In retrospect, it was a
bad idea to include the current request in the Pundit context. It bleeds
out everywhere and there are many contexts (in tests and models) where
we only have the current user, not the current request. The previous
commit got rid of the only two places where we used it.

app/models/post.rb

@@ -668,12 +668,12 @@ class Post < ApplicationRecord
 
         when /^-favgroup:(.+)$/i
           favgroup = FavoriteGroup.find_by_name_or_id!($1, CurrentUser.user)
-          raise User::PrivilegeError unless Pundit.policy!([CurrentUser.user, nil], favgroup).update?
+          raise User::PrivilegeError unless Pundit.policy!(CurrentUser.user, favgroup).update?
           favgroup&.remove!(self)
 
         when /^favgroup:(.+)$/i
           favgroup = FavoriteGroup.find_by_name_or_id!($1, CurrentUser.user)
-          raise User::PrivilegeError unless Pundit.policy!([CurrentUser.user, nil], favgroup).update?
+          raise User::PrivilegeError unless Pundit.policy!(CurrentUser.user, favgroup).update?
           favgroup&.add!(self)
 
         end
@@ -779,7 +779,7 @@ class Post < ApplicationRecord
 
     def add_favorite!(user)
       Favorite.add(post: self, user: user)
-      vote!("up", user) if Pundit.policy!([user, nil], PostVote).create?
+      vote!("up", user) if Pundit.policy!(user, PostVote).create?
     rescue PostVote::Error
     end
 
@@ -789,7 +789,7 @@ class Post < ApplicationRecord
 
     def remove_favorite!(user)
       Favorite.remove(post: self, user: user)
-      unvote!(user) if Pundit.policy!([user, nil], PostVote).create?
+      unvote!(user) if Pundit.policy!(user, PostVote).create?
     rescue PostVote::Error
     end
 
@@ -803,7 +803,7 @@ class Post < ApplicationRecord
     # Users who publicly favorited this post, ordered by time of favorite.
     def visible_favorited_users(viewer)
       favorited_users.order("favorites.id DESC").select do |fav_user|
-        Pundit.policy!([viewer, nil], fav_user).can_see_favorites?
+        Pundit.policy!(viewer, fav_user).can_see_favorites?
       end
     end
 
@@ -876,7 +876,7 @@ class Post < ApplicationRecord
     end
 
     def vote!(vote, voter = CurrentUser.user)
-      unless Pundit.policy!([voter, nil], PostVote).create?
+      unless Pundit.policy!(voter, PostVote).create?
         raise PostVote::Error.new("You do not have permission to vote")
       end