Issue #3604163: [3.0.x backport] Undefined array key in DomainSourceActionBase config form

Backports the fix from 3.x ([#3604163], commit b5d25299) to the 3.0.x branch.

DomainSourceActionBase::buildConfigurationForm() read the wrong config key (id instead of domain_id), so the Domain checkboxes lost the saved selection on edit and PHP warned about an undefined array key. Cherry-picked cleanly onto 3.0.x; the included DomainSourceActionFormTest passes there (and fails on the unpatched code).

Credits carried from the 3.x commit: das-peter (report + fix), mably (test).