Fix/chart loading use dashboard domain

[Adityashandilya555]

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [x] I have manually tested the changes proposed in this pull request.
• [ ] 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 #526 .

Please open a new issue if there isn't an existing issue yet.

Description of Changes

• Added DASHBOARD_BASEURL configuration variable using dashboard domain
• Changed OPENWISP_MONITORING_API_BASEURL to use DASHBOARD_BASEURL instead of API_BASEURL
• Chart loading now uses the same domain as the dashboard interface

Screenshot

Please include any relevant screenshots.