Add spec for opting-out of notification types (#2341)

specs/0110-notifications-opt-out.md

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+- Feature Name: Opt-out of specific notification types
+- Created: 2021-10-21
+- Associated Issue: #2064
+
+# Summary
+
+Add ability to opt-out of specific notification types.
+
+# Motivation
+
+The number of notifications users receive can be high, which can sometimes cause the most important notifications to be overlooked. Having the ability to opt-out of specific notifications would allow user to only see the ones they care about most.
+
+# Feature explanation
+
+In user settings, a list of notification types the user is subscribed to appears. By default, all options are on, i.e. users are subscribed to all notification types. By removing the check (and saving the settings), the user gets unsubscribed from the selected notification type.
+
+Users cannot opt-out of notifications that are manually sent.
+
+# Mockup
+
+![](0110/mockup.png)