Intent to Ship: <link rel=preload> and Link: rel=preload support on Nightly and Early Beta

[Honza Bambas]

** Summary **
"<link rel=preload>" HTML tag and "Link: rel=preload" HTTP response
header are a web technology allowing web developers to tell the browser
to early load and pre-cache resources that will be used by the currently
loading web page, specifically scripts, stylesheets, images, fonts and
fetch (for `fetch()` and `XMLHttpRequest`) early during the page load. 
This feature has been newly written in Gecko and replaces a preexisting
implementation that we have never enabled by default.

** Status in Firefox **
This has been undergoing stability dogfooding on Nightly for couple
weeks, Product Integrity Testing and Performance Evaluation are pending.

** Preferences **
The preference to flip this on is `network.preload`.

** Release plan **
- I'm about to enable it in this cycle, Firefox 78, for all users of
Nightly and then let it ride to Early Beta only.
- After PI and PE is done with good results we plan to do a gradual
release on Beta and Release using a study.

** Other browsers **
All other browsers do support this feature by default for at least the
most used resource types.

** Resources **
Specification: https://w3c.github.io/preload/
Tracking bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1583604
Project documentation with an overview explanation of the
implementation:
https://docs.google.com/document/d/11iAbml3EOyE8jxsB0-NLfxLWoDzIq-jCKJHAwLnM7z0/edit?usp=sharing
MDN: https://developer.mozilla.org/en-US/docs/Web/HTML/Preloading_content


-hb-