add script_id to user_levels and activities
* migration to add script id to user_levels and activities
* script to copy existing user_levels to new ones for each relevant
  script
* I don't think it's necessary to do the same for activities because we
  don't actually query activities in a script context for any user
  facing features (we use user_level where we need to do something like
  this)
never mind that, never touch the activities table
code to write user_levels with script_ids
Merge staging
cleanup
Merge pull request #647 from code-dot-org/user_level_script_2

add script_id to user_levels