(core) move client code to core

Summary:
This moves all client code to core, and makes minimal fix-ups to
get grist and grist-core to compile correctly.  The client works
in core, but I'm leaving clean-up around the build and bundles to
follow-up.

Test Plan: existing tests pass; server-dev bundle looks sane

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2627

app/client/models/ConnectState.ts

@@ -0,0 +1,41 @@
+/**
+ * The ConnectStateManager helper class helps maintain the connection state. A disconnect goes
+ * through multiple stages, to inform the user of long disconnects while minimizing the disruption
+ * for short ones. This class manages these timings, and triggers ConnectState changes.
+ */
+import {Disposable, Observable} from 'grainjs';
+
+// Describes the connection state, which is shown as part of the notifications UI.
+// See https://grist.quip.com/X92IAHZV3uoo/Notifications
+export enum ConnectState { Connected, JustDisconnected, RecentlyDisconnected, ReallyDisconnected }
+
+export class ConnectStateManager extends Disposable {
+  // On disconnect, ConnectState changes to JustDisconnected. These intervals set how long after
+  // the disconnect ConnectState should change to other values.
+  public static timeToRecentlyDisconnected = 5000;
+  public static timeToReallyDisconnected = 30000;
+
+  public readonly connectState = Observable.create<ConnectState>(this, ConnectState.Connected);
+
+  private _timers: Array<ReturnType<typeof setTimeout>> = [];
+
+  public setConnected(yesNo: boolean) {
+    if (yesNo) {
+      this._timers.forEach((t) => clearTimeout(t));
+      this._timers = [];
+      this._setState(ConnectState.Connected);
+    } else if (this.connectState.get() === ConnectState.Connected) {
+      this._timers = [
+        setTimeout(() => this._setState(ConnectState.RecentlyDisconnected),
+                   ConnectStateManager.timeToRecentlyDisconnected),
+        setTimeout(() => this._setState(ConnectState.ReallyDisconnected),
+                   ConnectStateManager.timeToReallyDisconnected),
+      ];
+      this._setState(ConnectState.JustDisconnected);
+    }
+  }
+
+  private _setState(state: ConnectState) {
+    this.connectState.set(state);
+  }
+}