Frontend: Hide secret creation elements from users who cannot create secrets

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Proposal

Hide buttons, links, and menu items that lead to screens used to create, edit, or delete secrets from users without permission to do so (when they have read permission but not write permission).

1. provide/inject a canCreateSecret boolean
   • this could eventually link up to a backend permission check like can?(current_user, :create_secret, parent)
2. add some v-if="canCreateSecret"s to the relevant UI elements