let the user set a grace period before having to use Touch ID

**Is your feature request related to a problem? Please describe.**
I have Touch ID enabled when I open the app. When switching apps, this can get quite annoying as I have to constantly scan my finger.
**Describe the solution you'd like**
Let the user set a time period (say, a minute) after closing before they have to scan their finger again.
**Describe alternatives you've considered**
I could turn it off (although I can't see the setting anymore), but that would somewhat compromise security.
**Additional context**
N/A.

The issue does not name files or tests. Start by locating the iOS Touch ID lock flow and the handling of app switching or backgrounding, then trace where authentication is enforced. Done means users can configure a grace period after closing or switching apps without disabling the security setting.

Written by the indexing model from the issue text.