Allow specifying user ID on case property for conditional alert recipients

[AddisonDunn]

Product Description

Jira: https://dimagi.atlassian.net/browse/USH-4811

For the "user specified via case property" recipient option for conditional alerts, this PR enables specifying a user ID (which can be for a web or mobile user) on a case property instead of just a mobile worker username.

Technical Summary

Small change.

Safety Assurance

Safety story

• Logic of change is well covered by automated tests
• Small change, confident in the safety of the logic
• Works on staging!

Automated test coverage

corehq.messaging.scheduling.tests.test_recipients:SchedulingRecipientTest.test_user_id_case_property_recipient

QA Plan

None planned.

Rollback instructions

• This PR can be reverted after deploy with no further considerations

Labels & Review

• Risk label is set correctly
• The set of people pinged as reviewers is appropriate for the level of risk of the change

[AddisonDunn]

If you'd read the testing section before now: I actually just learned conditional alerts only send when first created (not upon subsequent saves). Was able to test and confirm on staging.