Offline view of app details

Change the action button to say 'Download Later' with an outline style instead of the filled in style of 'download'. Selecting this flags the app to be downloaded automatically when the user is connected to the internet. A notification is sent to the user within the app in the updates view: https://gitlab.com/fdroid/fdroidclient/issues/840

We'll also need a state for this button after the user has selected it.