Auto merge of #6490 - samfoo:hsts-preload, r=jdm

Implement HSTS (preload-only)

Implement HSTS (preload-only) servo/servo#6105

* Downloads the HSTS preload list from the chromium repo (same as gecko), then convert it to a list appropriate for servo.
* Reads the preload list when creating a resource task, and implements STS for those domains.

Still todo:

* Read Strict-Transport-Security headers from servers and add details to the in-memory HSTS list. (note: this requires hyper or servo to implement an STS header struct. Hyper seems like the appropriate location, so I will create an issue/PR there soon). The work for this is nearly done with the exception of adding a new ControlMsg and the new header.
* Persist HSTS list to disk with known hosts (perhaps a different issue should be raised for this?)

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