FEATURE: backend support for user-selectable components

* FEATURE: backend support for user-selectable components

* fix problems with previewing default theme

* rename preview_key => preview_theme_id

* omit default theme from child themes dropdown and try a different fix

* cache & freeze stylesheets arrays

db/migrate/20180710172959_disallow_multi_levels_theme_components.rb

@@ -0,0 +1,68 @@
+class DisallowMultiLevelsThemeComponents < ActiveRecord::Migration[5.2]
+  def up
+    @handled = []
+    top_parents = DB.query("
+      SELECT parent_theme_id, child_theme_id
+      FROM child_themes
+      WHERE parent_theme_id NOT IN (SELECT child_theme_id FROM child_themes)
+    ")
+
+    top_parents.each do |top_parent|
+      migrate_child(top_parent, top_parent)
+    end
+
+    if @handled.size > 0
+      execute("
+        DELETE FROM child_themes
+        WHERE parent_theme_id NOT IN (#{top_parents.map(&:parent_theme_id).join(", ")})
+      ")
+    end
+
+    execute("
+      UPDATE themes
+      SET user_selectable = false
+      FROM child_themes
+      WHERE themes.id = child_themes.child_theme_id
+      AND themes.user_selectable = true
+    ")
+
+    default = DB.query_single("SELECT value FROM site_settings WHERE name = 'default_theme_id'").first
+    if default
+      default_child = DB.query("SELECT 1 AS one FROM child_themes WHERE child_theme_id = ?", default.to_i).present?
+      execute("DELETE FROM site_settings WHERE name = 'default_theme_id'") if default_child
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+
+  private
+
+  def migrate_child(parent, top_parent)
+    unless already_exists?(top_parent.parent_theme_id, parent.child_theme_id)
+      execute("
+        INSERT INTO child_themes (parent_theme_id, child_theme_id, created_at, updated_at)
+        VALUES (#{top_parent.parent_theme_id}, #{parent.child_theme_id}, now(), now())
+      ")
+    end
+
+    @handled << [top_parent.parent_theme_id, parent.parent_theme_id, parent.child_theme_id]
+
+    children = DB.query("
+      SELECT parent_theme_id, child_theme_id
+      FROM child_themes
+      WHERE parent_theme_id = :child", child: parent.child_theme_id
+    )
+
+    children.each do |child|
+      unless @handled.include?([top_parent.parent_theme_id, child.parent_theme_id, child.child_theme_id])
+        migrate_child(child, top_parent)
+      end
+    end
+  end
+
+  def already_exists?(parent, child)
+    DB.query("SELECT 1 AS one FROM child_themes WHERE child_theme_id = :child AND parent_theme_id = :parent", child: child, parent: parent).present?
+  end
+end