MSC3381: Polls (mk II) (#3381)

* Polls (mk II)

* m.poll.start

* m.poll.* types

* Allow multiple selections

* Give moderators access to closing polls

* Mention freeform edits

* Mention how edits are a security issue

* Enforce minimum and maximum length

* Rename open and secret

* Mention message pinning

* Update unstable definition for polls

* Rework responses into aggregations/annotations

* Note how answer IDs should be unique

* Mention MSC3523

* Revert change to annotations

* Mention how to handle invalid closure events

* Only count the first closure

* Add push rules

* spelling

* Clarify the extensible events structure

* Fix wording of answer handling in start event

* Clarify how the push rules work, and add one for ending polls

* WIP rewrite of polls to consider content blocks system

* Finish the polls rewrite

* Remove notifications

* Update links and MSC1767 behaviour

* Remove excess artifact

* Note UISI problems in potential issues section

* Note why MSC3523 is a bad idea

* Update proposals/3381-polls.md

Co-authored-by: Maxime NATUREL <[email protected]>

* Update MSC

* Update proposals/3381-polls.md

Co-authored-by: Kévin Commaille <[email protected]>

* Typo: event layering

* Minor clarifications from non-blocking review feedback

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Co-authored-by: Maxime NATUREL <[email protected]>
Co-authored-by: Kévin Commaille <[email protected]>