Redirect logged-in users away from register and login pages

[Peter2552]

Fixes #341

This pull request prevents logged-in users from accessing the registration and login pages.

Changes made

• Added a redirection in PytitionUserCreateView to prevent logged-in users from viewing /petition/register/.
• Created a CustomLoginView to redirect logged-in users away from /petition/login/.
• Updated urls.py to use the custom login view.

Expected behavior

• Anonymous users: can access /petition/register/ and /petition/login/.
• Logged-in users: are redirected to the home page (/petition/).

Testing

Tested locally with Docker:

• Confirmed redirection works correctly for logged-in users.
• Verified that registration and login work as expected for anonymous users.

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Thanks for the opportunity to contribute to Pytition!

[Peter2552]

PS: While testing this, I implemented a simple logout route locally for debugging.
It’s not part of this PR, but I can open a new issue if you’d like that functionality officially added.