post query builder: fix incompatibilities with Rails 6.1.

* Rename the `#negate` and `#and` methods that we monkey patch into
  ActiveRecord::Relation. These methods are now defined in Rails 6.1, but
  they shadow our methods and have slightly different behavior.
* Fix a call to `invert`. It no longer accepts an argument.

app/logical/concerns/searchable.rb

@@ -10,12 +10,13 @@ module Searchable
     1 + params.values.map { |v| parameter_hash?(v) ? parameter_depth(v) : 1 }.max
   end
 
-  def negate(kind = :nand)
+  def negate_relation
-    unscoped.where(all.where_clause.invert(kind).ast)
+    unscoped.where(all.where_clause.invert.ast)
   end
 
   # XXX hacky method to AND two relations together.
-  def and(relation)
+  # XXX Replace with ActiveRecord#and (cf https://github.com/rails/rails/pull/39328)
+  def and_relation(relation)
     q = all
     q = q.where(relation.where_clause.ast) if relation.where_clause.present?
     q = q.joins(relation.joins_values + q.joins_values) if relation.joins_values.present?

app/logical/post_query_builder.rb

@@ -92,8 +92,8 @@ class PostQueryBuilder
   def metatags_match(metatags, relation)
     metatags.each do |metatag|
       clause = metatag_matches(metatag.name, metatag.value, quoted: metatag.quoted)
-      clause = clause.negate if metatag.negated
+      clause = clause.negate_relation if metatag.negated
-      relation = relation.and(clause)
+      relation = relation.and_relation(clause)
     end
 
     relation