Add API support for webhooks

[matijagrcic]

The Fizzy webhook management isn't exposed as a API but currently only available through the HTML web interface.
Would it be possible to expose it as an API, then the whole process of creating the board, adding columns and webhooks can be automated.

Method	Endpoint	Description
GET	/:account_slug/boards/:board_id/webhooks.json	List all webhooks for a board
GET	/:account_slug/boards/:board_id/webhooks/:id.json	Get a specific webhook
POST	/:account_slug/boards/:board_id/webhooks.json	Create a new webhook
PUT	/:account_slug/boards/:board_id/webhooks/:id.json	Update a webhook (URL cannot be changed)
DELETE	/:account_slug/boards/:board_id/webhooks/:id.json	Delete a webhook
POST	/:account_slug/boards/:board_id/webhooks/:id/activation.json	Reactivate a webhook

 PERMITTED_ACTIONS = %w[
    card_assigned
    card_closed
    card_postponed
    card_auto_postponed
    card_board_changed
    card_published
    card_reopened
    card_sent_back_to_triage
    card_triaged
    card_unassigned
    comment_created
  ].freeze