ts: Migrate setup.js to typescript.

Also added global type definition for `get_offset_to_window`
helper function.

web/src/setup.ts

@@ -0,0 +1,35 @@
+import $ from "jquery";
+
+import * as blueslip from "./blueslip";
+import * as loading from "./loading";
+import {page_params} from "./page_params";
+import * as util from "./util";
+
+// Miscellaneous early setup.
+$(() => {
+    if (util.is_mobile()) {
+        // Disable the tutorial; it's ugly on mobile.
+        page_params.needs_tutorial = false;
+    }
+
+    page_params.page_load_time = Date.now();
+
+    // Display loading indicator.  This disappears after the first
+    // get_events completes.
+    if (!page_params.needs_tutorial) {
+        loading.make_indicator($("#page_loading_indicator"), {
+            abs_positioned: true,
+        });
+    }
+
+    $.fn.get_offset_to_window = function () {
+        return this[0].getBoundingClientRect();
+    };
+
+    $.fn.expectOne = function () {
+        if (blueslip && this.length !== 1) {
+            blueslip.error("Expected one element in jQuery set", {length: this.length});
+        }
+        return this;
+    };
+});