Correct time zone of API dates

[AndrewKvalheim]

Checklist

• I have read the Contribution & Best practices Guide.
• My branch is up-to-date with the upstream master branch.
  • For simplicity of implementation this is based on Use JSON date serializer #3089 instead.
• The tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate).

Short description of what this resolves

Dates returned in the API have the time zone set to UTC instead of the conference time zone, resulting in an incorrect time.

Changes proposed in this pull request

Set the time zone to the conference time zone.

(Note that 6ce2e66 is the only commit unique to this pull request; the prior two are proposed in #3089.)

Concerns

I’ve kept this separate from #3089 since it might lead to deeper issues with time handling.

• Is a serializer the correct place to address this? Should it be done in EventSchedule#start_time instead?
• Does this share a root cause with CFP open check should respect conference timezone #2769?