User clicks "Login with LinkedIn"
        ↓
Your app redirects to LinkedIn OAuth URL (with client_id)
        ↓
User logs in + grants permission
        ↓
LinkedIn redirects back with ?code=TEMP_CODE
        ↓
Your app calls get_access_token(TEMP_CODE)
        ↓
LinkedIn returns access_token
        ↓
Your app stores token in session
        ↓
Now you can post to LinkedIn API!