[feature] Suggests next available subnet #29

[pranshustuff]

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [x] I have manually tested the changes proposed in this pull request.
• [x] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [ ] I have updated the documentation.

Reference to Existing Issue

Closes #29.

Description of Changes

Function clean() now calculates next available subnet and adds it as a suggestion in the error message. Added an API endpoint to get next available subnet for a particular subnet. And modified tests.

NOTE: This PR does not add the auto-fill feature like the IP address form. The scope of this PR is limited to showing the next available subnet along with the error messages above the subnet field.

Screenshot

https://github.com/user-attachments/assets/70756ba5-9322-4346-a5a5-60c69bbd5cfa