Get Shopify OAuth connect URL

Initiate the Shopify OAuth flow for a store. Shopify is a connect-only platform: the connected account does not publish social posts, it powers the Blogs API (/v1/accounts/{accountId}/blogs) and the Products API (/v1/accounts/{accountId}/products). Returns an authUrl to redirect the merchant to; after they approve the install, Shopify redirects their browser to Zernio's callback, the account is created on the profile (platform shopify), and the browser is redirected to redirect_url (or the Zernio dashboard when omitted). Requested scopes are read_content, write_content, read_products and write_products (content and products only; no customer or order data). Connecting the same profile to a store again refreshes the stored token in place, and is how a store connected under the older content-only grant picks up product access.