Firefox 74 rel notes and other tasks

[chrisdavidmills]

Trello

https://trello.com/c/Z62Dhuao/246-firefox-74-release-notes

Acceptance criteria

1. • Firefox 74 release notes are updated per DDNs and DDCs.
2. • Experimental Features page is updated per above.
3. • platform-status is updated accordingly.
4. • MDN content roadmap trello cards updated accordingly.
5. • Hacks blogpost written about Fx 74 new features
6. • Put links to the Fx hacks blog post on the rel notes.

[chrisdavidmills]

Draft of Fx 74 blog post draft written: https://docs.google.com/document/d/1sz4525EgcqzW4MiENPo40jplcNDSAFXcZV--uajP6Xc/edit#

[chrisdavidmills]

Platform status updated to show feature-policy support in 74:

mozilla/platform-status#600