Polls push rules

[kerryarchibald]

• adds polls push rule ids from MSC3930
• adds function to retrieve a user's rule and kind by id getPushRuleAndKindById - used here

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

---

This change is marked as an internal change (Task), so will not be included in the changelog.

[andybalaam]

Looks good, thanks!