Possibility to define a time period after witch synchronized files are deleted locally

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**Is your feature request related to a problem? Please describe.**
One of the reason to have a cloud storage would be, I guess, to be able to clean up the limited memory of devices like phones/tablets. I would find it useful to set a maximum time for which to hold local copies of files for automatically synchronized folders.

**Describe the solution you'd like**
Something like a month for example would be enough to hold local copies of images taken with my phone, everything older should be deleted locally and kept just on my cloud storage.

**Describe alternatives you've considered**
Perhaps a toggle for some images I want to always keep locally, would be nice. But that's not a deal breaker, cos you could always copy them to another local folder, where the sync is not deleting old files.