Currently we load server-side code for our blocks by requiring each relevant file and registering blocks immediately.

The first part is fine; the second should potentially be done on init instead, if only to set an example for block implementers. (We can guarantee that register_block_type will be defined when we call it; other plugins won't be able to, and so should probably wait until init.)

The initialization was partly done this way in the original PR that added this code, but if we want to do this, then it should probably be done with a single callback for each block instead.