Some solace: you are not alone. This malware is currently injected on 284 stores, according to last night's scan.
You should run a thorough scan of your server to find any backdoors they may have planted. See my opensource scanner @ https://github.com/gwillem/magento-malware-scanner or a commercial version @ https://sansec.io.
You should also conduct a root cause analysis, otherwise you will likely have the same problem again in two weeks (20% of merchants get reinfected after the first time, see https://gwillem.gitlab.io/2018/11/12/merchants-struggle-with-magecart-reinfections/)
In general, you should search for requests containing "adminer", "phpmyadmin", "cms/block", "theme/design_config/save", and find other requests from the same IP addresses.