Add exit pages multiple branches

[thomasiles]

WIP: Use ExitPage model when available in Condition model

Trello card: https://trello.com/c/gZGRBU8Y/3172-change-add-edit-delete-exit-page-to-work-with-new-style-exit-page-model

First parts of using ExitPage model instead of condition attributes to model exit pages.

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[github-actions]

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