Improve logic around last seen value of applications in the Console

[ryaplots]

Summary

Make this logic behind determining the lastSeen value of an application more performant by doing it in the backend and then return one final value to the frontend.

Current Situation

The logic for determining the last seen value of an application is all done in the frontend reducer.
The lastSeen value of applications in the Console is a combination of the end devices last_seen_at and the latest uplink received. The lastSeen is updated every time one these values is more recent than the other.

Desired Situation

This logic can be done in the backend to make it much more performant.

Code of Conduct

• I agree to follow TTN's Community Code of Conduct.

[KrishnaIyer]

@ryaplots: Could you add some details to the issue when you have time?