Use a secure secret key for authentication & sessions

[haydenrou]

• Gather the SecretKey from OS env
• Generate some sort of CSRF token via the openssl package?
• Document this in the readme for local development
• enable cookie.Secure in production environments

[haydenrou]

See #34 for the initial auth PR and #37 for the session PR

[haydenrou]

From Session docs: https://pkg.go.dev/github.com/gorilla/sessions@v1.2.2#section-readme

// Note: Don't store your key in your source code. Pass it via an
// environmental variable, or flag (or both), and don't accidentally commit it
// alongside your code. Ensure your key is sufficiently random - i.e. use Go's
// crypto/rand or securecookie.GenerateRandomKey(32) and persist the result.