refs https://github.com/TryGhost/Team/issues/1277
- `data-cache` service has a `.set(key, data, lifetime)` method that will store the data under the key and sets a timeout that will remove the data when the lifetime expires
- data can be retrieved with `.get(key)`
- allows for components to cache data for use when re-rendering without having to worry about keeping track of their state and it's expiration manually somewhere else
- moved caching concern out of the `members-activity` service and into the latest-member-activity dashboard component which is the one that cares about it's data and cache lifetime
- frees the `members-activity` service up to be more generic as it's no longer tied to the dashboard component's concerns
- component switched to using a task rather than a promise so it is automatically cancelled if it's destroyed before data fetching is complete
