sentry: Avoid importing rate_limiting at topic level.

This fixes an issue where update-prod-static would crash with the following exception:

2021-07-19 03:59:24,601 upgrade-zulip-stage-2: Building static assets...
Traceback (most recent call last):
  File "./tools/update-prod-static", line 27, in <module>
    os.chdir(settings.DEPLOY_ROOT)
  File "/home/zulip/deployments/2021-07-19-03-58-39/zulip-py3-venv/lib/python3.6/site-packages/django/conf/__init__.py", line 82, in __getattr__
    self._setup(name)
  File "/home/zulip/deployments/2021-07-19-03-58-39/zulip-py3-venv/lib/python3.6/site-packages/django/conf/__init__.py", line 69, in _setup
    self._wrapped = Settings(settings_module)
  File "/home/zulip/deployments/2021-07-19-03-58-39/zulip-py3-venv/lib/python3.6/site-packages/django/conf/__init__.py", line 170, in __init__
    mod = importlib.import_module(self.SETTINGS_MODULE)
  File "/usr/lib/python3.6/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 994, in _gcd_import
  File "<frozen importlib._bootstrap>", line 971, in _find_and_load
  File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 665, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 678, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "./tools/../zproject/settings.py", line 20, in <module>
    from .computed_settings import *  # noqa: F401,F403 isort: skip
  File "./tools/../zproject/computed_settings.py", line 1186, in <module>
    from .sentry import setup_sentry
  File "./tools/../zproject/sentry.py", line 12, in <module>
    from zerver.lib.request import get_request_notes
  File "./tools/../zerver/lib/request.py", line 28, in <module>
    import zerver.lib.rate_limiter as rate_limiter
  File "./tools/../zerver/lib/rate_limiter.py", line 14, in <module>
    from zerver.models import UserProfile
  File "./tools/../zerver/models.py", line 26, in <module>
    from django.contrib.auth.models import AbstractBaseUser, PermissionsMixin, UserManager
  File "/home/zulip/deployments/2021-07-19-03-58-39/zulip-py3-venv/lib/python3.6/site-packages/django/contrib/auth/models.py", line 3, in <module>
    from django.contrib.auth.base_user import AbstractBaseUser, BaseUserManager
  File "/home/zulip/deployments/2021-07-19-03-58-39/zulip-py3-venv/lib/python3.6/site-packages/django/contrib/auth/base_user.py", line 48, in <module>
    class AbstractBaseUser(models.Model):
  File "/home/zulip/deployments/2021-07-19-03-58-39/zulip-py3-venv/lib/python3.6/site-packages/django/db/models/base.py", line 108, in __new__
    app_config = apps.get_containing_app_config(module)
  File "/home/zulip/deployments/2021-07-19-03-58-39/zulip-py3-venv/lib/python3.6/site-packages/django/apps/registry.py", line 253, in get_containing_app_config
    self.check_apps_ready()
  File "/home/zulip/deployments/2021-07-19-03-58-39/zulip-py3-venv/lib/python3.6/site-packages/django/apps/registry.py", line 136, in check_apps_ready
    raise AppRegistryNotReady("Apps aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet.
Traceback (most recent call last):
  File "/home/zulip/deployments/2021-07-19-03-58-39/scripts/lib/upgrade-zulip-stage-2", line 220, in <module>
    subprocess.check_call(["./tools/update-prod-static"], preexec_fn=su_to_zulip)
  File "/usr/lib/python3.6/subprocess.py", line 311, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['./tools/update-prod-static']' returned non-zero exit status 1.

zproject/sentry.py

@@ -9,7 +9,6 @@ from sentry_sdk.integrations.sqlalchemy import SqlalchemyIntegration
 from sentry_sdk.utils import capture_internal_exceptions
 
 from version import ZULIP_VERSION
-from zerver.lib.request import get_request_notes
 
 if TYPE_CHECKING:
     from sentry_sdk._types import Event, Hint
@@ -23,7 +22,7 @@ def add_context(event: "Event", hint: "Hint") -> Optional["Event"]:
             return None
     from django.conf import settings
 
-    from zerver.lib.request import get_current_request
+    from zerver.lib.request import get_current_request, get_request_notes
     from zerver.models import get_user_profile_by_id
 
     with capture_internal_exceptions():