reindex-textual-data: Provide a tool to reindex all text indices.

The script is added to upgrade steps for 20.04 and Buster because
those are the upgrades that cross glibc 2.28, which is most
problematic.  It will also be called out in the upgrade notes, to
catch those that have already done that upgrade.

docs/production/upgrade-or-modify.md

@@ -288,7 +288,16 @@ instructions for other supported platforms.
     /home/zulip/deployments/current/scripts/setup/upgrade-postgresql
     ```
 
-5. Finally, we need to reinstall the current version of Zulip, which
+5. Ubuntu 20.04 has a different version of the low-level glibc
+   library, which affects how PostgreSQL orders text data (known as
+   "collations"); this corrupts database indexes that rely on
+   collations.  Regenerate the affected indexes by running:
+
+   ```
+   /home/zulip/deployments/current/scripts/setup/reindex-textual-data --force
+   ```
+
+6. Finally, we need to reinstall the current version of Zulip, which
    among other things will recompile Zulip's Python module
    dependencies for your new version of Python and rewrite Zulip's
    full-text search indexes to work with the upgraded dictionary
@@ -445,7 +454,16 @@ instructions for other supported platforms.
 6. [Upgrade to the latest Zulip release](#upgrading-to-a-release), now
    that your server is running a supported operating system.
 
-7. As root, finish by verifying the contents of the full-text indexes:
+7. Debian Buster has a different version of the low-level glibc
+   library, which affects how PostgreSQL orders text data (known as
+   "collations"); this corrupts database indexes that rely on
+   collations.  Regenerate the affected indexes by running:
+
+   ```
+   /home/zulip/deployments/current/scripts/setup/reindex-textual-data --force
+   ```
+
+8. As root, finish by verifying the contents of the full-text indexes:
 
     ```
     /home/zulip/deployments/current/manage.py audit_fts_indexes