models: fix deprecated errors[:base] << "message" calls.

Replace the idiom `errors[:base] << "message"` with
`errors.add(:base, "message")`. The former is deprecated in Rails 6.1.

app/models/post.rb

@@ -490,7 +490,7 @@ class Post < ApplicationRecord
 
       invalid_tags.each do |tag|
         tag.errors.messages.each do |attribute, messages|
-          warnings[:base] << "Couldn't add tag: #{messages.join(';')}"
+          warnings.add(:base, "Couldn't add tag: #{messages.join(';')}")
         end
       end
 
@@ -976,7 +976,7 @@ class Post < ApplicationRecord
   module DeletionMethods
     def expunge!
       if is_status_locked?
-        self.errors.add(:is_status_locked, "; cannot delete post")
+        errors.add(:is_status_locked, "; cannot delete post")
         return false
       end
 
@@ -1082,11 +1082,11 @@ class Post < ApplicationRecord
     def copy_notes_to(other_post, copy_tags: NOTE_COPY_TAGS)
       transaction do
         if id == other_post.id
-          errors.add :base, "Source and destination posts are the same"
+          errors.add(:base, "Source and destination posts are the same")
           return false
         end
         unless has_notes?
-          errors.add :post, "has no notes"
+          errors.add(:post, "has no notes")
           return false
         end
 
@@ -1357,8 +1357,7 @@ class Post < ApplicationRecord
   module ValidationMethods
     def post_is_not_its_own_parent
       if !new_record? && id == parent_id
-        errors[:base] << "Post cannot have itself as a parent"
-        false
+        errors.add(:base, "Post cannot have itself as a parent")
       end
     end
 
@@ -1372,7 +1371,7 @@ class Post < ApplicationRecord
     end
 
     def uploader_is_not_limited
-      errors[:uploader] << uploader.upload_limit.limit_reason if uploader.upload_limit.limited?
+      errors.add(:uploader, uploader.upload_limit.limit_reason) if uploader.upload_limit.limited?
     end
 
     def added_tags_are_valid
@@ -1382,12 +1381,12 @@ class Post < ApplicationRecord
       if new_general_tags.present?
         n = new_general_tags.size
         tag_wiki_links = new_general_tags.map { |tag| "[[#{tag.name}]]" }
-        self.warnings[:base] << "Created #{n} new #{(n == 1) ? "tag" : "tags"}: #{tag_wiki_links.join(", ")}"
+        warnings.add(:base, "Created #{n} new #{(n == 1) ? "tag" : "tags"}: #{tag_wiki_links.join(", ")}")
       end
 
       new_artist_tags.each do |tag|
         if tag.artist.blank?
-          self.warnings[:base] << "Artist [[#{tag.name}]] requires an artist entry. \"Create new artist entry\":[/artists/new?artist%5Bname%5D=#{CGI.escape(tag.name)}]"
+          warnings.add(:base, "Artist [[#{tag.name}]] requires an artist entry. \"Create new artist entry\":[/artists/new?artist%5Bname%5D=#{CGI.escape(tag.name)}]")
         end
       end
     end
@@ -1398,7 +1397,7 @@ class Post < ApplicationRecord
 
       if unremoved_tags.present?
         unremoved_tags_list = unremoved_tags.map { |t| "[[#{t}]]" }.to_sentence
-        self.warnings[:base] << "#{unremoved_tags_list} could not be removed. Check for implications and try again"
+        warnings.add(:base, "#{unremoved_tags_list} could not be removed. Check for implications and try again")
       end
     end
 
@@ -1409,21 +1408,21 @@ class Post < ApplicationRecord
       return if tags.any?(&:artist?)
       return if Sources::Strategies.find(source).is_a?(Sources::Strategies::Null)
 
-      self.warnings[:base] << "Artist tag is required. \"Create new artist tag\":[/artists/new?artist%5Bsource%5D=#{CGI.escape(source)}]. Ask on the forum if you need naming help"
+      warnings.add(:base, "Artist tag is required. \"Create new artist tag\":[/artists/new?artist%5Bsource%5D=#{CGI.escape(source)}]. Ask on the forum if you need naming help")
     end
 
     def has_copyright_tag
       return if !new_record?
       return if has_tag?("copyright_request") || tags.any?(&:copyright?)
 
-      self.warnings[:base] << "Copyright tag is required. Consider adding [[copyright request]] or [[original]]"
+      warnings.add(:base, "Copyright tag is required. Consider adding [[copyright request]] or [[original]]")
     end
 
     def has_enough_tags
       return if !new_record?
 
       if tags.count(&:general?) < 10
-        self.warnings[:base] << "Uploads must have at least 10 general tags. Read [[howto:tag]] for guidelines on tagging your uploads"
+        warnings.add(:base, "Uploads must have at least 10 general tags. Read [[howto:tag]] for guidelines on tagging your uploads")
       end
     end
   end