Share limits

It would be nice to be able to impose some limits on read-write shares, such as

* max individual file size
* max total size of uploaded files
* max number of files
* option to disable creating/uploading directories

These would be useful if you're sharing the link with people you don't trust _that much_ to not fill up your entire storage.

They could work either globally(/per-volume) or per-share:

* global options only: every share has the same limits (this could also include a max lifetime for shares)
* per-share only: lacks admin control
* both: shares have maximum limits but can be further restricted per-share if needed

An existing alternative would be to have a dedicated volume for shares, but some situations might require more granular control, and it would be slightly inconvenient to have to move files between volumes before/after sharing.