user_settings: Add web_mark_read_on_scroll_policy field.

This is a backend change that will help support the new
`Mark messages as read on scroll` user setting.

zerver/tests/test_realm.py

@@ -1237,6 +1237,7 @@ class RealmAPITest(ZulipTestCase):
             default_view=["recent_topics", "all_messages"],
             emojiset=[emojiset["key"] for emojiset in RealmUserDefault.emojiset_choices()],
             demote_inactive_streams=UserProfile.DEMOTE_STREAMS_CHOICES,
+            web_mark_read_on_scroll_policy=UserProfile.WEB_MARK_READ_ON_SCROLL_POLICY_CHOICES,
             user_list_style=UserProfile.USER_LIST_STYLE_CHOICES,
             desktop_icon_count_display=[1, 2, 3],
             notification_sound=["zulip", "ding"],