Simplify and fix newtab's ESLint configuration

The newtab ESLint configuration is still largely set up as if it was being managed in a separate repository. Additionally the way it is set up causes confusion and complications.

I am working on some clean up which will:

• Remove extending the configuration from eslint:recommended and plugin:mozilla/recommended - these are not required since the newtab directory inherits from the top-level .eslintrc.js of the source directory.
  • This also means we don't have to redo the application of test configurations to xpcshell & browser tests.
• Stops marking everything as a module. This was causing issues such as:
  • enabling no-unused-vars at the global scope on the head.js files for tests,
  • not enabling rules when they should be enabled as they don't apply to modules.
• Collapses the configurations from the content-src and test directories into browser/components/newtab/.eslintrc.js
  • This centralises everything and makes it easier to know what applies.
• Disables no-insecure-url on a line/file basis rather than in the main configuration (the general idea is to do this).
• Fixes some ESLint issues that are raised due to the fixed configuration.