Intent to ship: ETP strict mode shims for content-blocked resources (aka SmartBlock)

[Thomas Wisniewski]

As of Firefox 87, I intend to enable ETP shims by default on desktop
and Android. ETP shims are work-arounds for website breakage which is
caused by blocking specific scripts in ETP strict mode and private browsing
mode (see the intent to prototype for more context). Shims for the
following scripts will be enabled in 87:
- Ads by Google
- BmAuth by 9c9media
- Eluminate (coremetrics.com)
- Google Analytics (and its Tag Manager and e-commerce plugins)
- Google IMA3
- Google Publisher Tags
- Rambler
- Rich Relevance

Note that shims for the Facebook SDK and Ad Safe Protected's Google IMA
adapter will remain nightly-only and will not be enabled for Firefox 87, as
they require further UX work (
https://bugzilla.mozilla.org/show_bug.cgi?id=1661330).

Bug to turn on by default:
https://bugzilla.mozilla.org/show_bug.cgi?id=1693386

This feature was previously discussed in this "Intent to prototype"
thread: https://groups.google.com/g/mozilla.dev.platform/c/28eWGe4s5a8