UX: Make Uncategorized category less confusing

* Adds warnings to the "Edit Category" dialog
* Doesn't hide the "Security" tab on the "Edit Category" dialog anymore. Instead, it shows an explanation why permissions can't be changed.
* Makes the category name translatable
* Hides the category name from the edit dialog (it can be customized by overriding the translation)
* Creates a translation override if the category has been renamed in the past

db/migrate/20190304170931_migrate_uncategorized_category_name.rb

@@ -0,0 +1,10 @@
+class MigrateUncategorizedCategoryName < ActiveRecord::Migration[5.2]
+  def change
+    execute <<~SQL
+      INSERT INTO translation_overrides (locale, translation_key, value, created_at, updated_at)
+      SELECT '#{I18n.locale}', 'uncategorized_category_name', name, now(), now()
+      FROM categories
+      WHERE id = #{SiteSetting.uncategorized_category_id} AND LOWER(name) <> 'uncategorized';
+    SQL
+  end
+end