Check that config is valid before saving

[Nutomic]

It turns out that you can currently set a config that is completely invalid (eg wrong braces), and Lemmy will happily save it, reload the config and then stop working. This adds a simple check to make sure that the config can be parsed before saving.

There are also some other problems with updating the config like this:

• If any error happens, it is simply reported as couldnt_update_site. Would be good if this could be more specific, eg indicating that the given config is invalid (and in which place), or that the file is not writable.
• It would be nice if we could pretty print the new config, but thats not supported by deser-hjson crate.
• Some config changes (e.g. enabling federation) require a restart of Lemmy. I tried to do that by caling System::exit() in save_config_file(), but it didnt seem to work. Would be good to mention in lemmy-ui that a restart is necessary.