[Proposed Feature] Last modified entry in the web interface

[calebrob6]

Description of Feature:
I'd love to see a last modified datetime for each file/repo on the web interface.

What value is this feature adding to Source Cooperative?
This would let me quickly see if the data in a repo has been updated recently. E.g. I just looked at https://source.coop/planet/disasterdata to check on the new data that Planet has released for the wildfires in France. I want to know whether they updated the main catalog.json also, and a Last modified: <sometime earlier today> would let me parse this at a glance. It would also be helpful to see as a signal of activity. If I visit a repo and it hasn't been modified in a few years, then I know it is likely not maintained.

[jedsundwall]

This should also be available programmatically. Whatever object-level metadata we display should also be available as a json blob via a predictable URL

[jedsundwall]

Note also that objects aren't "modified" but rewritten to object storage. In any event, we should make it clear when any object has been written or re-written, and we should make that clear at the product level as well.