Migrate plugin to React and implement initial version of new plugin

[orballo]

Migration:

• Update dependencies
• Wire up React
• Wire up MST
• Wire up styled-components
• Wire up grommet
• Config Webpack
• Config Babel
• Create PHP function to map a MST snapshot with the settings

Initial implementation:

• Create new settings
• Remove old settings functions
• Write activation function
• Write deactivation function
• Write update function
• Write uninstall function
• Build Frontity page
• Build Settings page

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The script added to the admin site will run every time a page is loaded, even if they are not the pages of the plugin. We should find a way to make the script load only in the needed pages, and if it's not possible the script should prevent React from running.

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The current wp_pwa_settings should be named frontity_settings. We need to be careful not to loose the old settings, and copy the values to the new ones before deleting them. We should look if there is a plugin_update hook of some kind to this purpose.

[frontibotito]

🎉 This issue has been resolved in version 1.10.0 🎉

The release is available on GitHub release

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