CSP fixer: shorter progress message
CSP fixer: ignore special dirs when processing (in particular .bower-git, .git)
CSP fixer: don't create backups of original sources by default

1) They are largely useless during normal development:
- CSP fixing is typically applied to vendored sources, which can be recovered in other ways;
- user's own sources can be typically recovered from source control;
- even if the above doesn't apply, the source can be reconstructed from the newly generated parts with some manual work.
2) Creating backups polutes the disk and slows down CSP fixing.
CSP Fixer: throttle concurrent fixing threads to unfreeze the UI

This is a stop-gap measure; a proper fix should be using a ServiceWorker. TODO added.
Address comments
Merge pull request #3805 from ussuri/csp-fixer-improvements

CSP fixer improvements