Allow to customize artifacts retention period

[benoit74]

With #917 we have introduced the possibility to upload artifacts at the end of task

With #938 we introduce the fact that we would like the Zimfarm to re-use these artifacts to make some enhance task processing

This means that we will probably need / want to keep artifacts for longer than the default value (30 days) on some occasions.

We should be able to configure this retention in the recipe configuration UI.

It would be nice to also be able to see the expiration date of a given artifact in the task output (next to the download artifact button for instance, which could be disabled if artifact has expired).