DEV: Introduce experimental type: objects theme setting (#25538)

Why this change?

This commit introduces an experimental `type: objects` theme setting
which will allow theme developers to store a collection of objects as
JSON in the database. Currently, the feature is still in development and
this commit is simply setting up the ground work for us to introduce the
feature in smaller pieces.

What does this change do?

1. Adds a `json_value` column as `jsonb` data type to the `theme_settings` table.
2. Adds a `experimental_objects_type_for_theme_settings` site setting to
   determine whether `ThemeSetting` records of with the `objects` data
   type can be created.
3. Updates `ThemeSettingsManager` to support read/write access from the
   `ThemeSettings#json_value` column.

app/models/theme_setting.rb

@@ -5,8 +5,12 @@ class ThemeSetting < ActiveRecord::Base
 
   has_many :upload_references, as: :target, dependent: :destroy
 
+  TYPES_ENUM =
+    Enum.new(integer: 0, float: 1, string: 2, bool: 3, list: 4, enum: 5, upload: 6, objects: 7)
+
   validates_presence_of :name, :theme
-  validates :data_type, numericality: { only_integer: true }
+  before_validation :objects_type_enabled
+  validates :data_type, inclusion: { in: TYPES_ENUM.values }
   validates :name, length: { maximum: 255 }
 
   after_save :clear_settings_cache
@@ -24,7 +28,7 @@ class ThemeSetting < ActiveRecord::Base
   end
 
   def self.types
-    @types ||= Enum.new(integer: 0, float: 1, string: 2, bool: 3, list: 4, enum: 5, upload: 6)
+    TYPES_ENUM
   end
 
   def self.acceptable_value_for_type?(value, type)
@@ -37,6 +41,9 @@ class ThemeSetting < ActiveRecord::Base
       value.is_a?(TrueClass) || value.is_a?(FalseClass)
     when self.types[:list]
       value.is_a?(String)
+    when self.types[:objects]
+      # TODO: This is a simple check now but we want to validate the default objects agianst the schema as well.
+      value.is_a?(Array)
     else
       true
     end
@@ -62,6 +69,15 @@ class ThemeSetting < ActiveRecord::Base
       types[:bool]
     end
   end
+
+  private
+
+  def objects_type_enabled
+    if self.data_type == ThemeSetting.types[:objects] &&
+         !SiteSetting.experimental_objects_type_for_theme_settings
+      self.data_type = nil
+    end
+  end
 end
 
 # == Schema Information
@@ -75,4 +91,5 @@ end
 #  theme_id   :integer          not null
 #  created_at :datetime         not null
 #  updated_at :datetime         not null
+#  json_value :jsonb
 #