Support Github organisations with IP restrictions

[sentry-io]

As per the following error in Sentry, we don't currently support GH organisations that have an IP filter on their API access. I believe the only way to solve this would be to add an internet gateway with a static IP address that we can publish for our customers.

Sentry Issue: FLAGSMITH-API-4YJ

GithubException: 403 {"message": "Although you appear to have the correct authorization credentials, the `<organisation name>` organization has an IP allow list enabled, and your IP address is not permitted to access this resource.", "documentation_url": "https://docs.github.com/rest/reference/apps#create-an-installation-access-token-for-an-app", "status": "403"}
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  File "integrations/github/views.py", line 71, in wrapper
    return func(*args, **kwargs)
  File "integrations/github/views.py", line 239, in fetch_repositories
    data = fetch_github_repositories(
  File "integrations/github/client.py", line 205, in fetch_github_repositories
    headers: dict[str, str] = build_request_headers(installation_id)
  File "integrations/github/client.py", line 40, in build_request_headers
    else generate_token(
  File "integrations/github/client.py", line 62, in generate_token
    token = auth.token