drafts: Delete old drafts on page load, not modal launch.

This fixes a bad user experience where clicking to open the drafts
modal causes the number of drafts that you're looking at to suddenly
change.

frontend_tests/node_tests/drafts.js

@@ -201,8 +201,6 @@ test("remove_old_drafts", () => {
 });
 
 test("format_drafts", ({override, mock_template}) => {
-    override(drafts, "remove_old_drafts", noop);
-
     function feb12() {
         return new Date(1549958107000); // 2/12/2019 07:55:07 AM (UTC+0)
     }

static/js/drafts.js

@@ -389,7 +389,6 @@ export function launch() {
         });
     }
 
-    remove_old_drafts();
     const drafts = format_drafts(draft_model.get());
     render_widgets(drafts);
 
@@ -551,6 +550,8 @@ export function set_initial_element(drafts) {
 }
 
 export function initialize() {
+    remove_old_drafts();
+
     window.addEventListener("beforeunload", () => {
         update_draft();
     });