Adding a one-time email sign up to my start of my app
<p>I want to add a one-time, mandatory email sign up at the beginning of my basic app. So, if the user has never used the app before, it will send them to the "sign up" page where they will enter an email, hit "submit" and then be sent to the home page of the app. If the user has already opened the app before, and therefore already entered an email, they should be sent straight to my app's home page.</p>

<p>How can I go about this?</p>

<p>Also, as an addition, how can I store all the emails that have been submitted through my app in a spreadsheet or the like?</p>