(core) Nice summary table IDs

Summary:
Changes auto-generated summary table IDs from e.g. `GristSummary_6_Table1` to `Table1_summary_A_B` (meaning `Table1` grouped by `A` and `B`). This makes it easier to write formulas involving summary tables, make API requests, understand logs, etc.

Because these don't encode the source table ID as reliably as before, `decode_summary_table_name` now uses the summary table schema info, not just the summary table ID. Specifically, it looks at the type of the `group` column, which is `RefList:<source table id>`.

Renaming a source table renames the summary table as before, and now renaming a groupby column renames the summary table as well.

Conflicting table names are resolved in the usual way by adding a number at the end, e.g. `Table1_summary_A_B2`. These summary tables are not automatically renamed when the disambiguation is no longer needed.

A new migration renames all summary tables to the new scheme, and updates formulas using summary tables with a simple regex.

Test Plan:
Updated many tests to use the new style of name.

Added new Python tests to for resolving conflicts when renaming source tables and groupby columns.

Added a test for the migration, including renames in formulas.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3508

sandbox/grist/schema.py

@@ -15,7 +15,7 @@ import six
 
 import actions
 
-SCHEMA_VERSION = 30
+SCHEMA_VERSION = 31
 
 def make_column(col_id, col_type, formula='', isFormula=False):
   return {