Fix code scanning alert no. 21: URL redirection from remote source

[TreyWW]

Fixes https://github.com/TreyWW/MyFinances/security/code-scanning/21

To fix the problem, we need to validate the redirect_url to ensure it is a safe and allowed URL. We can use Django's url_has_allowed_host_and_scheme function to check that the URL is safe to redirect to. This function ensures that the URL does not contain an explicit host name and is within the allowed hosts.

1. Import the url_has_allowed_host_and_scheme function from django.utils.http.
2. Modify the redirect_to_login function to validate the redirect_url using url_has_allowed_host_and_scheme.
3. If the redirect_url is not valid, redirect to a default safe URL (e.g., the home page).

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