Specify room version 10: knock_restricted and int power levels

[turt2live]

Incorporates #1082 (and moves it into the room version sections)
Describes MSC3787 - knock_restricted
Describes MSC3667 - int power levels
Describes MSC3604 - room version 10

Reviewer: This is somewhat set up for commit-by-commit review given diff size, though note the few things pointed out below. #1082 has been incorporated with a merge commit, followed by commits to move it to the right place and such, for example.

Other notable changes with this PR:

• Some of the fragments/weights for the room versions were wrong - this is fixed because we're in the area.
• There is a bug where **[New in this version]** (and similar) appears as a literal in the room version spec. clarify federation Authorization header an add destination property #1067 fixes this as it ran into the same problem.
  • Fixed: PR merged
• The auth rules for integer power levels are based upon the Dendrite implementation rather than the Synapse implementation given the Dendrite impl was used as evidence for the purposes of FCP. The Synapse implementation only needs minor edits to get in line, however.
• I would love to use multiple spaces in the example stringy power level values, however markdown collapses sequences of whitespace down to a single character, even in inline code blocks.

Preview: https://pr1099--matrix-spec-previews.netlify.app

[turt2live]

@richvdh this should be ready for re-review, with the commits since review being self-contained.

[richvdh]

lgtm!