Add avg update interval to item stats

[Nakroma]

I'm submitting a...

• Bug report
• Feature request
• Documentation issue or request

Current behavior

Currently you have to estimate the item update intervals yourself by looking at the graphs and guessing.

Expected behavior

Could be a nice improvement to just see the average update interval under the last updated section (could also be good to see if somethings wrong - e.g. if the update interval is 1h and last updated is 5h ago)

Minimal reproduction of the problem

Environment

Browser:

• Chrome (desktop) version:
• Chrome (Android) version:
• Chrome (iOS) version:
• Firefox version:
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For development issues:

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Additional Description