[qa] Switched JSS and CSS linting to prettier

[nemesifier]

Checklist

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

Reference to Existing Issue

Testing the switch from jshint and stylelint to prettier done in openwisp/openwisp-utils#425.

[coveralls]

coverage: 98.261%. remained the same
when pulling 5899095 on prettier
into 211bebc on master.