quotas can now be specified using the `quotas_json` option of `component file_sharing`
section in settings. This must be a dict where:
- `users` key contains default quotas for all users
- `admins` key contains quotas for administrators (not implemented yet)
- `jids` contain bare JID to quota mapping, to have user-specific quota
The value can be either a int for quota in bytes, or a case insensitive string with an
optional multiplier symbol (e.g. "500 Mio"). `None` can be used for explicit unlimited
quota (which is the default is `users` is not set).
When a file size is too big for quota, upload is refused with an error message indicating
allowed quota, used space, and the size of the file that user wants to upload.
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