Use named routes instead of url helper with paths

[Michael-Stokoe]

Changes in this PR:

• Changed instances of url() . '/some/path' to route('named.route') instead. (Allows for us to change the routes of the package incase there are conflicts with pre-existing application routes).
• Moved routes to dedicated file, to clean up the service provider
• Moved namespaced resources to /src/ directory.
• Moved resources to root directory.
• Moved migrations to root directory.

[Michael-Stokoe]

Fixes this:

https://github.com/appzcoder/laravel-admin/issues/74