(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/moment.py

@@ -4,7 +4,6 @@ import marshal
 from time import time
 import bisect
 import os
-import moment_parse
 import iso8601
 import six
 from six.moves import zip
@@ -13,7 +12,6 @@ from six.moves import zip
 ZoneRecord = namedtuple("ZoneRecord", ("name", "abbrs", "offsets", "untils"))
 
 # moment.py mirrors core functionality of moment-timezone.js
-# moment.py includes function parse, located and documented in moment_parse.py
 # Documentation: http://momentjs.com/timezone/docs/
 
 EPOCH = datetime(1970, 1, 1)
@@ -67,10 +65,6 @@ def date_to_ts(date, timezone=None):
   ts = (date - DATE_EPOCH).total_seconds()
   return ts if not timezone else ts - timezone.offset(ts * 1000).total_seconds()
 
-# Calls parse from moment_parse.py
-def parse(date_string, parse_format, zonelabel='UTC'):
-  return moment_parse.parse(date_string, parse_format, zonelabel)
-
 # Parses a datetime in the ISO format, YYYY-MM-DDTHH:MM:SS.mmmmmm+HH:MM. Most parts are optional;
 # see https://pypi.org/project/iso8601/ for details. Returns a timestamp in seconds.
 def parse_iso(date_string, timezone=None):