Consider allowing size-time tradeoff for uploaded media.

This issue has been migrated from [#8732](https://github.com/matrix-org/synapse/issues/8732).

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Allow size-time tradeoff for files Matrix is very useful to transfer files, but most sysadmins don't want to set a high size limit for uploads, to avoid excessive storage requirements. Both concerns could be served if the sysadmin could set a function from size to retention time (in the most basic case a simple t=f(s)=constant/s). This would mesh well with the request for self-destroying messages. Messages might be kept longer on some servers than on others, and in that case they could still be called up for download.