(core) Initial webhooks implementation

Summary:
See https://grist.quip.com/VKd3ASF99ezD/Outgoing-Webhooks

- 2 new DocApi endpoints: _subscribe and _unsubscribe, not meant to be user friendly or publicly documented. _unsubscribe should be given the response from _subscribe in the body, e.g:

```
$ curl -X POST -H "Authorization: Bearer 8fd4dc59ecb05ab29ae5a183c03101319b8e6ca9" "http://localhost:8080/api/docs/6WYa23FqWxGNe3AR6DLjCJ/tables/Table2/_subscribe" -H "Content-type: application/json" -d '{"url": "https://webhook.site/a916b526-8afc-46e6-aa8f-a625d0d83ec3", "eventTypes": ["add"], "isReadyColumn": "C"}'
{"unsubscribeKey":"3246f158-55b5-4fc7-baa5-093b75ffa86c","triggerId":2,"webhookId":"853b4bfa-9d39-4639-aa33-7d45354903c0"}
$ curl -X POST -H "Authorization: Bearer 8fd4dc59ecb05ab29ae5a183c03101319b8e6ca9" "http://localhost:8080/api/docs/6WYa23FqWxGNe3AR6DLjCJ/tables/Table2/_unsubscribe" -H "Content-type: application/json" -d '{"unsubscribeKey":"3246f158-55b5-4fc7-baa5-093b75ffa86c","triggerId":2,"webhookId":"853b4bfa-9d39-4639-aa33-7d45354903c0"}'
{"success":true}
```

- New DB entity Secret to hold the webhook URL and unsubscribe key
- New document metatable _grist_Triggers subscribes to table changes and points to a secret to use for a webhook
- New file Triggers.ts processes action summaries and uses the two new tables to send webhooks.
- Also went on a bit of a diversion and made a typesafe subclass of TableData for metatables.

I think this is essentially good enough for a first diff, to keep the diffs manageable and to talk about the overall structure. Future diffs can add tests and more robustness using redis etc. After this diff I can also start building the Zapier integration privately.

Test Plan: Tested manually: see curl commands in summary for an example. Payloads can be seen in https://webhook.site/#!/a916b526-8afc-46e6-aa8f-a625d0d83ec3/0b9fe335-33f7-49fe-b90b-2db5ba53382d/1 . Great site for testing webhooks btw.

Reviewers: dsagal, paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3019

sandbox/gen_js_schema.py

@@ -13,6 +13,7 @@ _ts_types = {
   "PositionNumber": "number",
   "Ref":            "number",
   "RefList":        "['L', ...number[]]|null",    # Non-primitive values are encoded
+  "ChoiceList":     "['L', ...string[]]|null",
   "Text":           "string",
 }
 

sandbox/grist/migrations.py

@@ -795,3 +795,15 @@ def migration23(tdset):
     add_column('_grist_DocInfo', 'documentSettings', 'Text'),
     actions.UpdateRecord('_grist_DocInfo', 1, {'documentSettings': '{"locale":"en-US"}'})
   ])
+
+
+@migration(schema_version=24)
+def migration24(tdset):
+  return tdset.apply_doc_actions([
+    actions.AddTable('_grist_Triggers', [
+      schema.make_column("tableRef", "Ref:_grist_Tables"),
+      schema.make_column("eventTypes", "ChoiceList"),
+      schema.make_column("isReadyColRef", "Ref:_grist_Tables_column"),
+      schema.make_column("actions", "Text"),  # JSON
+    ]),
+  ])

sandbox/grist/schema.py

@@ -15,7 +15,7 @@ import six
 
 import actions
 
-SCHEMA_VERSION = 23
+SCHEMA_VERSION = 24
 
 def make_column(col_id, col_type, formula='', isFormula=False):
   return {
@@ -236,6 +236,14 @@ def schema_create_actions():
     ]),
 
 
+    # Triggers subscribing to changes in tables
+    actions.AddTable("_grist_Triggers", [
+      make_column("tableRef", "Ref:_grist_Tables"),
+      make_column("eventTypes", "ChoiceList"),
+      make_column("isReadyColRef", "Ref:_grist_Tables_column"),
+      make_column("actions", "Text"),  # JSON
+    ]),
+
     # All of the ACL rules.
     actions.AddTable('_grist_ACLRules', [
       make_column('resource',     'Ref:_grist_ACLResources'),