settings: Add backend to change allow_edit_history to integer field.

This commit implements the backend of migrating the
`allow_edit_history` setting to
`message_edit_history_visibility_policy`.
This allows organizations, to have an intermediate setting to
view only the "Moves" history of the messages.

We still pass `realm_allow_edit_history` in `/register` response
though for older clients with its value being set depending on the
value of `realm_message_edit_history_visibility_policy`. We set
`realm_allow_edit_history` to `False` if the
`realm_message_edit_history_visibility_policy` is "None", and
`True` for "Moves only" or "All" message edit history.

Fixes part of #21398.

Co-authored-by: Shlok Patel <shlokcpatel2001@gmail.com>
Co-authored-by: Tim Abbott <tabbott@zulip.com>

zerver/lib/events.py

@@ -97,6 +97,7 @@ from zerver.models.linkifiers import linkifiers_for_realm
 from zerver.models.realm_emoji import get_all_custom_emoji_for_realm
 from zerver.models.realm_playgrounds import get_realm_playgrounds
 from zerver.models.realms import (
+    MessageEditHistoryVisibilityPolicyEnum,
     get_corresponding_policy_value_for_group_setting,
     get_realm_domains,
     get_realm_with_settings,
@@ -474,6 +475,18 @@ def fetch_initial_state_data(
         )
 
         state["realm_empty_topic_display_name"] = Message.EMPTY_TOPIC_FALLBACK_NAME
+
+        state["realm_allow_edit_history"] = (
+            realm.message_edit_history_visibility_policy
+            != MessageEditHistoryVisibilityPolicyEnum.none.value
+        )
+
+        state["realm_message_edit_history_visibility_policy"] = (
+            MessageEditHistoryVisibilityPolicyEnum(
+                realm.message_edit_history_visibility_policy
+            ).name
+        )
+
     if want("realm_user_settings_defaults"):
         realm_user_default = RealmUserDefault.objects.get(realm=realm)
         state["realm_user_settings_defaults"] = {}
@@ -1311,6 +1324,11 @@ def apply_event(
                     else state["server_jitsi_server_url"]
                 )
 
+            if field == "realm_message_edit_history_visibility_policy":
+                state["realm_allow_edit_history"] = (
+                    event["value"] != MessageEditHistoryVisibilityPolicyEnum.none.name
+                )
+
         elif event["op"] == "update_dict":
             for key, value in event["data"].items():
                 if key == "max_file_upload_size_mib":