Revert "migrations: Replace special chars in stream & user names with space."

This reverts commit acebd3a5e, as well as a subsequent fixup commit
0975bebac "quick fix: Fix migrations to be linear."

These changes need more work and thought before they're ready to
deploy on any large established Zulip server, such as zulipchat.com.
See discussion on #6534.

In place of the removed migration, leave behind a placeholder so
`manage.py migrate` doesn't get confused on installs where it was
already applied.

zerver/migrations/0127_disallow_chars_in_stream_and_user_name.py

@@ -1,39 +1,5 @@
 # -*- coding: utf-8 -*-
-from django.db import models, migrations
-from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
-from django.db.migrations.state import StateApps
-from typing import Text
-
-def remove_special_chars_from_streamname(apps, schema_editor):
-    # type: (StateApps, DatabaseSchemaEditor) -> None
-    Stream = apps.get_model('zerver', 'Stream')
-    NAME_INVALID_CHARS = ['*', '@', '`', '#']
-    for stream in Stream.objects.all():
-        if (set(stream.name).intersection(NAME_INVALID_CHARS)):
-            for char in NAME_INVALID_CHARS:
-                stream.name = stream.name.replace(char, ' ').strip()
-
-            while Stream.objects.filter(name__iexact=stream.name, realm=stream.realm).exists():
-                stream.name = stream.name + '^'
-                if len(stream.name) > 60:
-                    # extremely unlikely, so just do something valid
-                    stream.name = stream.name[-60:]
-            stream.save(update_fields=['name'])
-
-def remove_special_chars_from_username(apps, schema_editor):
-    # type: (StateApps, DatabaseSchemaEditor) -> None
-    UserProfile = apps.get_model('zerver', 'UserProfile')
-    NAME_INVALID_CHARS = ['*', '`', '>', '"', '@', '#']
-    for userprofile in UserProfile.objects.all():
-        if (set(userprofile.full_name).intersection(NAME_INVALID_CHARS)):
-            for char in NAME_INVALID_CHARS:
-                userprofile.full_name = userprofile.full_name.replace(char, ' ').strip()
-            userprofile.save(update_fields=['full_name'])
-
-        if (set(userprofile.short_name).intersection(NAME_INVALID_CHARS)):
-            for char in NAME_INVALID_CHARS:
-                userprofile.short_name = userprofile.short_name.replace(char, ' ').strip()
-            userprofile.save(update_fields=['short_name'])
+from django.db import migrations
 
 class Migration(migrations.Migration):
 
@@ -42,6 +8,8 @@ class Migration(migrations.Migration):
     ]
 
     operations = [
-        migrations.RunPython(remove_special_chars_from_streamname),
-        migrations.RunPython(remove_special_chars_from_username),
+        # There was a migration here, which wasn't ready for wide deployment
+        # and was backed out.  This placeholder is left behind to avoid
+        # confusing the migration engine on any installs that applied the
+        # migration.  (Fortunately no reverse migration is needed.)
     ]