Potential fix for code scanning alert no. 2: Incomplete URL substring sanitization

[M03ED]

Potential fix for https://github.com/PasarGuard/panel/security/code-scanning/2

To fix the problem, update the Discord webhook URL validation to robustly check the hostname using URL parsing rather than simple string matching. Specifically:

• Parse the URL using urllib.parse.urlparse.
• Verify that the scheme is HTTPS and the netloc/host is either exactly discord.com or a documented canonical subdomain (if allowed), such as discord.com (and optionally discordapp.com if relevant).
• Reject URLs that do not meet these requirements.
• Make these changes within class DiscordValidator, specifically in the validate_webhook method, replacing the string prefix check.

You'll need to:

• Import urlparse from urllib.parse (if not present).
• Adjust the method to parse the URL, handle possible errors (such as poorly formed URLs), and ensure the host and scheme are as required.
• Only accept the webhook URL if both the scheme and host match.

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