CSP fixer improvements

@umop 

* Ignore special directories (ones starting with a `.`)
* Do not create backup sources by default (see the comment in one of the commits in this PR as to why)
* Most importantly, throttle the number of concurrent fixing threads to unfreeze the UI somewhat. It's just a stop-gap solution for now; a proper one will use a service worker (there's a TODO in the code, but I'll file a bug as well).

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