ref(cells) Use signupLocalities in UI code

[markstory]

Update relocations and organization create to use the singupLocalities attribute that will be exposed in __initialData #118204.

These changes also remove relocationConfig as with signup_visible we can make relocationConfig redundant. I have also converted relocations from storing URLs in its state to using locality names. This aligns the implementation between relocations and org create.

Refs INFRENG-350

[linear-code]

INFRENG-350

[github-actions]

🚨 Warning: This pull request contains Frontend and Backend changes!

It's discouraged to make changes to Sentry's Frontend and Backend in a single pull request. The Frontend and Backend are not atomically deployed. If the changes are interdependent of each other, they must be separated into two pull requests and be made forward or backwards compatible, such that the Backend or Frontend can be safely deployed independently.

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[cursor]

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