(core) Barely working reference lists in frontend

Summary:
This makes it possible to set the type of a column to ReferenceList, but the UI is terrible

ReferenceList.ts is a mishmash of ChoiceList and Reference that sort of works but something about the CSS is clearly broken

ReferenceListEditor is just a text editor, you have to type in a JSON array of row IDs. Ignore the value that's present when you start editing. I can maybe try mashing together ReferenceEditor and ChoiceListEditor but it doesn't seem wise.
I think @georgegevoian should take over here. Reviewing the diff as it is to check for obvious issues is probably good but I don't think it's worth trying to land/merge anything.

Test Plan: none

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: georgegevoian

Differential Revision: https://phab.getgrist.com/D2914

sandbox/grist/functions/info.py

@@ -188,10 +188,10 @@ def ISREF(value):
   """
   Checks whether a value is a table record.
 
-  For example, if a column person is of type Reference to the People table, then ISREF($person)
-  is True.
-  Similarly, ISREF(People.lookupOne(name=$name)) is True. For any other type of value,
-  ISREF() would evaluate to False.
+  For example, if a column `person` is of type Reference to the `People` table,
+  then `ISREF($person)` is `True`.
+  Similarly, `ISREF(People.lookupOne(name=$name))` is `True`. For any other type of value,
+  `ISREF()` would evaluate to `False`.
 
   >>> ISREF(17)
   False
@@ -202,6 +202,25 @@ def ISREF(value):
   return isinstance(value, Record)
 
 
+def ISREFLIST(value):
+  """
+  Checks whether a value is a [`RecordSet`](#recordset),
+  the type of values in Reference List columns.
+
+  For example, if a column `people` is of type Reference List to the `People` table,
+  then `ISREFLIST($people)` is `True`.
+  Similarly, `ISREFLIST(People.lookupRecords(name=$name))` is `True`. For any other type of value,
+  `ISREFLIST()` would evaluate to `False`.
+
+  >>> ISREFLIST(17)
+  False
+  >>> ISREFLIST("Roger")
+  False
+
+  """
+  return isinstance(value, RecordSet)
+
+
 def ISTEXT(value):
   """
   Checks whether a value is text.