[UX] Use Javascript to complete block Admin label fields based on Display title

This is a follow up to https://github.com/backdrop/backdrop-issues/issues/1515 and https://github.com/backdrop/backdrop-issues/issues/1522

Users don't often care about the admin labels for their blocks, and would be perfectly happy if they were an exact match to the block title. Let's aid them along a little bit by copying the Display title into the Admin label field using Javascript.