feat(issue-details): Register priority and assignee UI flag (#122909)

Registers `organizations:issue-priority-assignee-ui` as an
organization-scoped FlagPole feature exposed to the frontend. The flag
remains off until rollout configuration enables it, preserving the
existing priority and assignee controls by default.

This registration must land before the dependent control redesign in
#122507.

Refs
[ISWF-3216](https://linear.app/getsentry/issue/ISWF-3216/update-priority-and-assignee-selectors).