Harmonize the affiliation constraints for the AB and TAG

[frivoal]

As discussed in #817, since there is no clear ongoing reason to justify the difference of treatment between the AB and TAG when affiliation changes cause two or more participants to share the same affiliation, this PR proposes to harmonize the rules for the two groups, by aligning on the more lenient one in terms of duration (the TAG's) while keeping details from the more specific one (the AB's) in terms of backstop.

Note: I have also switched the responsibility for running (if necessary) the verifiable random selection procedure from the Chair to the Team, as this is a purely administrative yet fiddly step, whose timing would now be tied to the election, which is already run by the Team. So that seemed simpler. We can of course switch back if there's a preference to that effect.

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[css-meeting-bot]

The Revising W3C Process CG just discussed https://github.com/w3c/process/pull/1167.

The full IRC log of that discussion

<brent> subtopic: https://github.com//pull/1167
<brent> Github https://github.com//pull/1167
<brent> Github: https://github.com//pull/1167
<Ian> Florian: This relates to harmonization of participation constraints between AB/TAG, and where the distinction was not useful.
<Ian> ...this pull request harmonizes on the "more lenient" approach.
<Ian> Florian: I did not go into detail about when the team would run the random procedure.
<Ian> ...for now ok to be unspecific, IMO.
<Ian> Hober: +1
<Ian> ....to the point that no timing detail needed for random procedure.
<tidoust> scribe+
<tidoust> Ian: Do "Affiliations" include substantial relationships? I don't remember what that encompasses
<tidoust> Florian: It covers with some nuance.
<tidoust> ... If you're a contractor with a primary affiliation, you're affiliated with that organization. If you have many customers, you're affiliated as an individual.
<Ian> https://www.w3.org/policies/process/#AB-TAG-constraints
<tidoust> Ian: Under "change of affiliation", there's a link to that section.
<tidoust> ... In other words, there's some relevant about change of affiliation that appears in a different section that might be more useful in *this* section.
<tidoust> ... Especially if they are about the constraints.
<tidoust> Florian: There's a link missing from that "change of affiliation" section to "primary affiliation". Orthogonal to the proposed change though.
<tidoust> ... Hmm, wait, we do link to it.
<Ian> Florian: might be useful to move text around; I'll have a look.
<Ian> Florian: Ian can open an issue later on making editorial improvements around change of affiliations.
<Ian> [No objections to merging this]
<tidoust> +1 from me and I approved the PR
<Ian> (3 Approvals and no objections)
<Ian> Resolved: Merge PR 1167