Upload router local content file

Required permission: Device command and configuration, Edit.<br/>Upload html file to allow it to be configured as the HTTPS server local content file for router configs. File must be HTML, under 4MB, and file name under 100 characters. Files are stored in a public bucket for configured routers to download. File should be attached as multipart/form-data. See https://starlink.readme.io/docs/local-content for example upload script.