buddy list: Fix and simplify up/down navigation.

This introduces a generic class called list_cursor to handle the
main details of navigating the buddy list and wires it into
activity.js.  It replaces some fairly complicated code that
was coupled to stream_list and used lots of jQuery.

The new code interacts with the buddy_list API instead of jQuery
directly.  It also persists the key across redraws, so we don't
lose our place when a focus ping happens or we type more characters.

Note that we no longer cycle to the top when we hit the bottom, or
vice versa.  Cycling can be kind of an anti-feature when you want to
just lay on the arrow keys until they hit the end.

The changes to stream_list.js here do not affect the left sidebar;
they only remove code that was used for the right sidebar.

.eslintrc.json

@@ -146,6 +146,7 @@
         "user_search": false,
         "buddy_data": false,
         "buddy_list": false,
+        "list_cursor": false,
         "activity": false,
         "invite": false,
         "colorspace": false,

frontend_tests/casper_tests/03-narrow.js

@@ -470,22 +470,23 @@ casper.waitForSelector('#user_presences .highlighted_user', function () {
 
 // Use arrow keys to navigate through suggestions
 casper.then(function () {
-    // Down: Cordelia -> Hamlet
-    casper.sendKeys('.user-list-filter', casper.page.event.key.Down, {keepFocus: true});
-    // Up: Hamlet -> Cordelia
-    casper.sendKeys('.user-list-filter', casper.page.event.key.Up, {keepFocus: true});
-    // Up: Cordelia -> aaron
-    casper.sendKeys('.user-list-filter', casper.page.event.key.Up, {keepFocus: true});
+    function arrow(key) {
+        casper.sendKeys('.user-list-filter',
+                        casper.page.event.key[key],
+                        {keepFocus: true});
+    }
+    arrow('Down'); // Cordelia -> Hamlet
+    arrow('Up'); // Hamlet -> Cordelia
+    arrow('Up'); // already at top
+    arrow('Down'); // Cordelia -> Hamlet
 });
 
-casper.waitForSelector('#user_presences li.highlighted_user [data-name="aaron"]', function () {
+casper.waitForSelector('#user_presences li.highlighted_user [data-name="King Hamlet"]', function () {
     casper.test.info('Suggestion highlighting - after arrow key navigation');
     casper.test.assertDoesntExist('#user_presences li.highlighted_user [data-name="Cordelia Lear"]',
         'User Cordelia Lear not is selected');
-    casper.test.assertDoesntExist('#user_presences li.highlighted_user [data-name="King Hamlet"]',
-        'User King Hamlet is not selected');
-    casper.test.assertExist('#user_presences li.highlighted_user [data-name="aaron"]',
-        'User aaron is selected');
+    casper.test.assertExist('#user_presences li.highlighted_user [data-name="King Hamlet"]',
+        'User King Hamlet is selected');
 });
 
 common.then_log_out();