(core) Migrate import code from data engine to Node

Summary:
Finishing imports now occurs in Node instead of the
data engine, which makes it possible to import into
on-demand tables. Merging code was also refactored
and now uses a SQL query to diff source and destination
tables in order to determine what to update or add.

Also fixes a bug where incremental imports involving
Excel files with multiple sheets would fail due to the UI
not serializing merge options correctly.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3046

app/server/lib/SQLiteDB.ts

@@ -544,6 +544,6 @@ async function createBackupFile(filePath: string, versionNum: number): Promise<s
  * Validate and quote SQL identifiers such as table and column names.
  */
 export function quoteIdent(ident: string): string {
-  assert(/^\w+$/.test(ident), `SQL identifier is not valid: ${ident}`);
+  assert(/^[\w.]+$/.test(ident), `SQL identifier is not valid: ${ident}`);
   return `"${ident}"`;
 }