install was redirecting to default baseUrl because `redirect->back()` uses the `site_url()` function

fix #53

- add `.env.example` and `INSTALL.md` to castopod bundle for installation docs
- update seeders to be silent on insert errors
- update install layout
- add manual config instructions when .env file is not writable
- fix eslint error in Charts.ts module

closes #32

- generate .env file to configure instance's environment
- add phpdotenv dependency to verify .env file
- add AppSeeder to call all required seeds at once
- add env and superadmin form views using form helpers

closes #2