migrations: Squash zerver migrations.

This has the impact of making rebuilding the database in a Zulip
development environment, or initializing a new production database,
dramatically faster.

This was generated by merging the output of `manage.py makemigrations`
with an empty migration.

Tested using `tools/rebuild-test-database` before and after this
change, and comparing the output of `pg_dump -d zulip_test` using
Git's diff comparison algorithm. Differences in that SQL dump include:

- The actual generated table contents, due to timestamps and the like;
  this is expected and unrelated to schema.

- Orders of fields within tables, which is not significant in SQL.

- IDs assigned to tables in the ContentType table, which is expected
  and not a problem with how that Django table is designed.

- Names of generated indexes and constraints; modern Django seems to
  abbreviate long names differently for these, and it's not obviously
  possible to configure those used by the `db_index` property. If
  necessarily, likely this can be converged via a migration filled
  with `IF EXISTS` rename operations like the one done in
  zerver/migrations/0246_message_date_sent_finalize_part2.py.

- Names of the ~3 sequences related to renamed tables:
  usertopic/mutedtopic, botconfigdata/botuserconfigdata,
  realmdomain/realmalias. Probably there's no action required here,
  but we could do rename operations if desired.

tools/semgrep-py.yml

@@ -72,6 +72,7 @@ rules:
       include:
         - "**/migrations"
       exclude:
+        - zerver/migrations/0001_squashed_0569.py
         - zerver/migrations/0032_verify_all_medium_avatar_images.py
         - zerver/migrations/0104_fix_unreads.py
         - zerver/migrations/0206_stream_rendered_description.py
@@ -93,6 +94,7 @@ rules:
       exclude:
         - analytics/migrations/0008_add_count_indexes.py
         - zerver/migrations/0001_initial.py
+        - zerver/migrations/0001_squashed_0569.py
         - zerver/migrations/0082_index_starred_user_messages.py
         - zerver/migrations/0083_index_mentioned_user_messages.py
         - zerver/migrations/0095_index_unread_user_messages.py