Allow dumping/cleaning the job history via a ketrew subcommand

[armish]

The pagination and filtering by job names using the web UI quickly becomes harder and harder as the number of (sometimes repetitive) job submissions get higher and higher. It would be great to provide a CLI-based option to dump the history (as a backup) and clean it to give the UI more breathing room.

I guess this is doable via some interaction with the underlying database, but doing it via ketrew would be better/easier.

[armish]

(assigned myself on purpose to improve my ketrew skills a bit ;))

[smondet]

You know you can "hide" older stuff from the UI right?

E.g. with the query:

(and (created-in-the-past (days 2)) (is-activated-by-user))