(core) Add unquarantine command to admin CLI

Summary:
Adds a CLI command to un-quarantine an active document. Also tweaks the
name of related environment variable to avoid a naming conflict.

Test Plan: Server test.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3583

app/gen-server/lib/DocWorkerMap.ts

@@ -141,6 +141,10 @@ class DummyDocWorkerMap implements IDocWorkerMap {
     // nothing to do
   }
 
+  public async removeDocGroup(docId: string): Promise<void> {
+    // nothing to do
+  }
+
   public getRedisClient() {
     return null;
   }
@@ -525,6 +529,10 @@ export class DocWorkerMap implements IDocWorkerMap {
     await this._client.setAsync(`doc-${docId}-group`, docGroup);
   }
 
+  public async removeDocGroup(docId: string): Promise<void> {
+    await this._client.delAsync(`doc-${docId}-group`);
+  }
+
   public getRedisClient(): RedisClient {
     return this._client;
   }

app/gen-server/lib/Housekeeper.ts

@@ -166,12 +166,12 @@ export class Housekeeper {
     // actions.
     //
     // Optionally accepts a `group` query param for updating the document's group prior
-    // to moving. This is useful for controlling which worker group the document is assigned
-    // a worker from.
+    // to moving. A blank string unsets the current group, if any. This is useful for controlling
+    // which worker group the document is assigned a worker from.
     app.post('/api/housekeeping/docs/:docId/assign', this._withSupport(async (req, docId, headers) => {
       const url = new URL(await this._server.getHomeUrlByDocId(docId, `/api/docs/${docId}/assign`));
       const group = optStringParam(req.query.group);
-      if (group) { url.searchParams.set('group', group); }
+      if (group !== undefined) { url.searchParams.set('group', group); }
       return fetch(url.toString(), {
         method: 'POST',
         headers,