message: add submit loading indicator
diff: @onClick={{this.manager.submitRegistration}}
  @disabled={{this.shouldDisableSubmitButton}}
  >
  + {{#if this.manager.submittingRegistration}}
  + <LoadingIndicator @inline=true />
  + {{else}}
  {{t 'registries.finalizeRegistrationModal.submit'}}
  + {{/if}}
  </OsfButton>
  <OsfButton
  data-test-back-button
  