(core) move DocUsageBanner to ext

Summary:
grist-ee build was failing since it didn't have a
DocUsageBanner implementation available. Made the implementation
added to monorepo available, since it will be useful to improve
the activation banner.

Test Plan: manaul

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: georgegevoian

Differential Revision: https://phab.getgrist.com/D3452

app/server/lib/dbUtils.ts

@@ -55,8 +55,12 @@ export async function getOrCreateConnection(): Promise<Connection> {
       // If multiple servers are started within the same process, we
       // share the database connection.  This saves locking trouble
       // with Sqlite.
-      return getConnection();
+      const connection = getConnection();
+      return connection;
     } catch (e) {
+      if (!String(e).match(/ConnectionNotFoundError/)) {
+        throw e;
+      }
       const connection = await createConnection();
       // When using Sqlite, set a busy timeout of 3s to tolerate a little
       // interference from connections made by tests. Logging doesn't show