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/migrations/0678_remove_realm_allow_edit_history.py

@@ -0,0 +1,16 @@
+# Generated by Django 5.0.10 on 2025-02-16 10:47
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0677_set_default_message_edit_history_visibility_policy"),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name="realm",
+            name="allow_edit_history",
+        ),
+    ]