This adds support for autocompleting issues that were recently viewed by
the current user when they use the search bar. This data is persisted in
Redis sorted sets as this provides the ability to automatically expire
the data for inactive users, keep the data sorted based on most recently
viewed first and easily keep a limited number of items per user expiring
the least recent ones first.

The recent item tracking is partly implemented as a generic solution as
this will quickly be followed up with merge requests and perhaps other
features people wish to quickly navigate to. The full generic
refactoring will happen when we implement the 2nd usage but for now the
most important thing is to include the data type in the Redis key so
that we don't need to migrate data when we make this generic later.

This feature is behind a feature flag per user for now so we can track
the performance implications in production.