(core) move home server into core

Summary: This moves enough server material into core to run a home server.  The data engine is not yet incorporated (though in manual testing it works when ported).

Test Plan: existing tests pass

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2552

app/common/ColumnGetters.ts

@@ -0,0 +1,26 @@
+/**
+ *
+ * An interface for accessing the columns of a table by their
+ * ID in _grist_Tables_column, which is the ID used in sort specifications.
+ * Implementations of this interface can be supplied to SortFunc to
+ * sort the rows of a table according to such a specification.
+ *
+ */
+export interface ColumnGetters {
+
+  /**
+   *
+   * Takes a _grist_Tables_column ID and returns a function that maps
+   * rowIds to values for that column.  Those values should be display
+   * values if available, drawn from a corresponding display column.
+   *
+   */
+  getColGetter(colRef: number): ((rowId: number) => any) | null;
+
+  /**
+   *
+   * Returns a getter for the manual sort column if it is available.
+   *
+   */
+  getManualSortGetter(): ((rowId: number) => any) | null;
+}