(core) New type conversion in the backend

Summary: This is https://phab.getgrist.com/D3205 plus some changes (https://github.com/dsagal/grist/compare/type-convert...type-convert-server?expand=1) that move the conversion process to the backend. A new user action ConvertFromColumn uses `call_external` so that the data engine can delegate back to ActiveDoc. Code for creating formatters and parsers is significantly refactored so that most of the logic is in `common` and can be used in different ways.

Test Plan: The original diff adds plenty of tests.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3240

sandbox/grist/functions/info.py

@@ -500,6 +500,14 @@ def N(value):
   return 0
 
 
+def CURRENT_CONVERSION(rec):
+  """
+  Special function used only when changing the type of a column.
+  Doesn't work in normal formulas.
+  """
+  return rec.gristHelper_Converted
+
+
 def NA():
   """
   Returns the "value not available" error, `#N/A`.

sandbox/grist/functions/schedule.py

@@ -1,12 +1,16 @@
 from datetime import datetime, timedelta
 import re
 from .date import DATEADD, NOW, DTIME
-from moment_parse import MONTH_NAMES, DAY_NAMES
 
 # Limit exports to schedule, so that upper-case constants like MONTH_NAMES, DAY_NAMES don't end up
 # exposed as if Excel-style functions (or break docs generation).
 __all__ = ['SCHEDULE']
 
+MONTH_NAMES = ['january', 'february', 'march', 'april', 'may', 'june', 'july', 'august',
+  'september', 'october', 'november', 'december']
+# Regex list of lowercase weekdays with characters after the first three made optional
+DAY_NAMES = ['sunday', 'monday', 'tuesday', 'wednesday', 'thursday', 'friday', 'saturday']
+
 def SCHEDULE(schedule, start=None, count=10, end=None):
   """
   Returns the list of `datetime` objects generated according to the `schedule` string. Starts at