Certs are always stored in PEM format

[fef1312]

JWT certificates that have been generated at initialization (development mode only) are always written to disk in PEM format, even if the configuration specified a .der extension. We could either write a complicated (and possibly error-prone) API for converting certificates to DER based on the file extension from the config, or just abandon DER support all together. I'll probably head for the latter approach since this platform is written for FreeTube and not meant to be run anywhere else, which means that designing the server to be configured in a particularily convenient manner is definitely not a priority.