Display expected build time in package details

[mizdebsk]

Koschei knows about build start and end times. It would be valuable to display expected build time in package details view - it could be for example weighted average running time for 10 or 5 last successful scratch builds (with more recent build having higher weights).

Having such information would help identifying packages which consume the most amount of Koji resources and perhaps should have static priority set to negative value, arch override adjusted, or even be excluded from Koschei.

[mizdebsk]

This is very closely related to #46 - the same materialized view could be re-used for storing average build times.