I removed the html block (only way to make page editable) and now the block is gone (from Installed Block Types list). How do I add this block again? The code is still there (concrete/blocks/html).
Ideally you should have reverted the page version rather than removing the block. That would have left the core block installed.
If you were on 5.6, there is an addon for restoring core blocks. Maybe you can look at what that does and do the same in 5.7. Take a backup first. That way you can revert the database if anything foes wrong.
If you were on 5.6, there is an addon for restoring core blocks. Maybe you can look at what that does and do the same in 5.7. Take a backup first. That way you can revert the database if anything foes wrong.