(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/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