(core) Ignore leading whitespace in formulas, and strip out leading '=' sign users might add

Summary:
This addresses two issues, differently:
- For a formula with leading whitespace, like " 1+1", it is stored as is, but
  is fixed to work (it should be valid Python, and whitespace is only stripped out
  at parsing time to avoid intentation errors caused by the way it gets parsed)
- For a formula with a leading equals-sign ("="), it is stripped out on the
  client side before the formula is stored. Grist documentation uses leading
  "=" to indicate formulas (because UI shows an "=" icon), and Excel formulas
  actually contain the leading "=", so it is a common mistake to include it.

Test Plan: Added new test cases

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3873

sandbox/grist/useractions.py

@@ -16,7 +16,7 @@ import actions
 import column
 import sort_specs
 import identifiers
-from objtypes import strict_equal, encode_object, decode_object
+from objtypes import strict_equal, encode_object
 import schema
 from schema import RecalcWhen
 import summary