Move the ImportController to its rightful place dradis_core

* Add a route for /import.json
* Adjust the JS files to load from the new /import.json
* Delay the loading of plugin data stores a few seconds to speed up startup
Simplify Plugin management

* Include Dradis::Core::Plugins::Base in the module instead of the Engine
* Provide a .plugin_name method so we can present that to the user in the UI
* Remove the need for plugins to register (we do that for them in the self.included)
* Adjust the ExportController to use the new plugin_name feature
Frontend - decrease delay in loading available plugins
Core - implement ImportController#filters, add routes tweak JS code