component file sharing: quotas implementation: 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.