(core) Implement UI for trigger formulas.

Summary:
- Implement UI with "Apply to new records" and "Apply on record changes"
  checkboxes, and options for selecting which changes to recalculate on.
- For consistency, always represent empty RefList as None
- Fix up generated SchemaTypes to remember that values are encoded.

Included test cases for the main planned use cases:
- Auto-filled UUID column
- Data cleaning
- NOW() formula for record's last-updated timestamp.
- Updates that depend on other columns.

Test Plan: Added a browser test.

Reviewers: jarek

Reviewed By: jarek

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D2885

sandbox/gen_js_schema.py

@@ -16,7 +16,7 @@ _ts_types = {
   "Int":            "number",
   "PositionNumber": "number",
   "Ref":            "number",
-  "RefList":        "number[]",
+  "RefList":        "['L', ...number[]]|null",    # Non-primitive values are encoded
   "Text":           "string",
 }
 
@@ -25,7 +25,7 @@ def get_ts_type(col_type):
   return _ts_types.get(col_type, "CellValue")
 
 def main():
-  print("""
+  print("""\
 /*** THIS FILE IS AUTO-GENERATED BY %s ***/
 // tslint:disable:object-literal-key-quotes
 

sandbox/grist/usertypes.py

@@ -457,7 +457,8 @@ class ReferenceList(BaseColumnType):
       assert value._table.table_id == self.table_id
       return objtypes.RecordList(value._row_ids, group_by=value._group_by, sort_by=value._sort_by)
     elif not value:
-      return []
+      # Represent an empty ReferenceList as None (also its default value). Formulas will see [].
+      return None
     return [Reference.do_convert(val) for val in value]
 
   @classmethod