Adds FeesService hooks, and token elementUid option

[kennethormandy]

Adds fee event hooks from ko-fees-event branch:

• kennethormandy\marketplace\services\FeesService
  • FeesService::EVENT_BEFORE_CALCULATE_FEES_AMOUNT
  • FeesService::EVENT_AFTER_CALCULATE_FEES_AMOUNT

Adds support for connecting new accounts from the front-end, and saving the platform account ID to elements other than the current User.

<form
  method="POST"
  action="{{ app.getRedirectUrl({ elementUid: team.uid }) }}">
  {{ csrfInput() }}
  {{ redirectInput(craft.app.request.url) }}

  <input type="submit" value="Connect" />
</form>