DEV: Increase max length of theme_field name column

Now that we have custom SCSS files with arbitrary names, it's easy to exceed 30 characters

db/migrate/20190423112954_increase_theme_field_name_length.rb

@@ -0,0 +1,5 @@
+class IncreaseThemeFieldNameLength < ActiveRecord::Migration[5.2]
+  def change
+    change_column :theme_fields, :name, :string, limit: 255
+  end
+end