Add an outgoing webhook alert and hide IFTTT from the new-alert UI.

[shadyvb]

Fixes XWPENG-56.

Adds a generic Outgoing Webhook alert (URL, POST or PUT, optional headers, JSON body with record-field placeholders). IFTTT is not offered when creating a new alert; existing IFTTT alerts still fire and remain editable. Maker recipes can be migrated by pointing a webhook at https://maker.ifttt.com/trigger/{event}/with/key/{key}.

Stacked on #1995.

Test plan

• New-alert type list includes Outgoing Webhook and omits IFTTT.
• Saving a webhook alert stores URL and method and shows “Outgoing Webhook” in the list.
• An existing IFTTT alert still shows Maker fields on inline edit.
• Ability create rejects ifttt; stream/get-alerts redacts webhook url and headers.