FIX: extracted theme JavaScripts for multisite (PR #6502)

NOTE: this includes a onceoff job to rebake all HTML theme fields, which will

  1. extract inline javascripts in themes, and
  2. correct the wrong theme-javascript url for multisite


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I intentionally picked update over update!, so that one failure will not throw off the entire job.

@xrav3nz I think we should use post deployment migration here instead as onceoff job is in the process of being pahsed out. Note that in migration we tend to avoid using ActiveRecord and prefer SQL directly. This is so because we don’t want future changes in Ruby code to break the migration when a new site is being set up.

To me this feels enormously urgent, I am merging in, please do a follow up commit to address @tgxworld’s concern regarding the once off.