Add voters count support (mastodon#11917)

* Add voters count to polls

* Add ActivityPub serialization and parsing of voters count

* Add support for voters count in WebUI

* Move incrementation of voters count out of redis lock

* Reword “voters” to “people”

Update changelog for 3.0.0rc1 (mastodon#11950)

Bump version to 2.9.3

Fix unconverted PRs in the changelog (mastodon#11155)

Bump version to 2.9.1 (mastodon#11143)

Bump version to 2.9.0 (mastodon#11074)

Bump version to 2.9.0rc2 (mastodon#11070)

Bump version to 2.9.0rc1 (mastodon#11004)

Bump version to 2.8.4

Bump version to 2.8.3