scripts: Tool to find the diff to an original settings.py prod template.

This hits the unauthenticated Github API to get the list of tags,
which is rate-limited to 60 requests per hour.  This means that the
tool can only be run 60 times per hour before it starts to exit with
errors, but that seems like a reasonable limit for the moment.

docs/production/upgrade-or-modify.md

@@ -133,6 +133,14 @@ suggest using that updated template to update
    section in `/etc/zulip/settings-new.py` and copy the setting from
    `settings.py` into there.
 
+   The following tool may help, by finding the most likely version of
+   the template that your `/etc/zulip/settings.py` was installed
+   using, and the differences that your file has from that:
+
+   ```
+   /home/zulip/deployments/current/scripts/setup/compare-settings-to-template
+   ```
+
    If there are settings which you cannot find documented in
    `/etc/zulip/settings-new.py`, check the [changelog][changelog] to see
    if they have been removed.

scripts/setup/compare-settings-to-template

@@ -0,0 +1,62 @@
+#!/usr/bin/env python3
+
+import difflib
+import os
+import re
+import sys
+
+BASE_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+sys.path.append(BASE_DIR)
+from scripts.lib.setup_path import setup_path
+
+setup_path()
+
+
+current_path = "/etc/zulip/settings.py"
+if len(sys.argv) >= 2:
+    current_path = sys.argv[1]
+
+print(f"Reading current configuration from {current_path}...")
+with open(current_path, "r") as f:
+    current_contents = f.read()
+
+import requests
+
+print("Fetching old versions of setting templates from Github...")
+templ = {}
+resp = requests.get("https://api.github.com/repos/zulip/zulip/tags")
+if resp.status_code != 200:
+    print(resp.content)
+    sys.exit(1)
+
+for tag in [t["name"] for t in resp.json()]:
+    if re.match(r"^\d+\.\d+(\.\d+)?$", tag):
+        print(f"  - {tag}")
+        resp = requests.get(
+            f"https://raw.githubusercontent.com/zulip/zulip/{tag}/zproject/prod_settings_template.py",
+        )
+        if resp.status_code == 200:
+            templ[tag] = resp.content.decode("utf-8")
+        else:
+            print("Failure: ")
+            print(resp)
+
+print("Computing minimal difference...")
+sequence_matchers = {}
+for version, contents in templ.items():
+    sm = difflib.SequenceMatcher(None, current_contents, contents)
+    sequence_matchers[version] = sm
+
+likely = sorted(sequence_matchers.keys(), key=lambda v: sequence_matchers[v].ratio(), reverse=True)
+print(f"Most likely version: {likely[0]}")
+
+
+diff = difflib.unified_diff(
+    templ[likely[0]].splitlines(keepends=True),
+    current_contents.splitlines(keepends=True),
+    fromfile="zulip-server-{version}/zproject/prod_settings_template.py",
+    tofile=current_path,
+)
+
+print()
+print("".join(diff), end="")