Fixes broken custom emoji in autocomplete when proxying to remote BE

As someone who often accesses their own instance via localhost dev pleroma instance, seeing broken emojis in autocomplete is very annoying. Problem is that custom emojis in autocomplete have domain-relative uris, so it's trying to find emojis on local machine which most likely don't have them (unless you sync them). Solution is to provide FE with information about what domain server is running on and prepending it to custom emojis. This would also make setting up secondary PleromaFE (like, running both stable FE on domain.tld and develop build on beta.domain.tld) much easier.