Add DiffTimestamps function

[carlinmack]

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Description

Please describe briefly your pull request.

At the moment we have toRelativeTime which produces a string like "in 2 minutes" or "2 minutes ago" from one timestamp.

In this PR I introduce diffTimestamps which produces a string like "2 minutes" from two time stamps.

Unfortunately I have had to write my own version of this as we are using an old version of Luxon. I'm not sure how much hassle it would be to upgrade Luxon.

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[carlinmack]

moving this code to invenio-jobs