(core) Removing GRIST_FORMULA_ASSISTANT flag

Summary:
A floating formula editor is available by default and in the basic setup allows just formula modification.
AI assistant is now an optional component of the floating editor and it is controlled by OPENAPI_KEY presence.
Env variable GRIST_FORMULA_ASSISTANT was removed, new feature flag HAS_FORMULA_ASSISTANT is derived from the presence of OPENAPI_KEY.

Also updated anonymous signup nudge. By default it displays only info that this feature is only for logged in users.

Test Plan: updated

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3987

test/nbrowser/gristUtils.ts

@@ -3014,7 +3014,13 @@ export function withEnvironmentSnapshot(vars: Record<string, any>) {
     // Test if the vars are already set, and if so, skip.
     if (Object.keys(vars).every(k => process.env[k] === vars[k])) { return; }
     oldEnv = new testUtils.EnvironmentSnapshot();
-    Object.assign(process.env, vars);
+    for(const key of Object.keys(vars)) {
+      if (vars[key] === undefined || vars[key] === null) {
+        delete process.env[key];
+      } else {
+        process.env[key] = vars[key];
+      }
+    }
     await server.restart();
   });
   after(async () => {