Editing of custom head script

This patch moves the custom_head_script column from CommunityCustomization to Community and makes it editable at edit_look_and_feel. After this patch, different custom_head_scripts for different locales are no longer supported.

IMPORTANT: The migration that moves the column will fail if there is a community with differing custom head scripts for different locales. The solution is to delete all scripts but one, which will become the master script. To accomplish different behaviors for different locales, the user should detect the locale in JavaScript.