starting from now, every commit should be cljfmt-fixed

processor: tidy up log msg and level; improve code style

- all messages start with a lower-case letter
- all webmention-related logging contains first 8-digit of its id
- use WARN level when something is deleted, not INFO
- use WARN level when exceptional cases happen, not ERROR
- get rid of emojis in log messages

utils: digest-short function for logging

verifier: tidy up log messages; improve code style

- use `cond` instead of nested `if`s
- same message format as processor's logs

webhook: tidy up log messages; send webhooks robustly

- use same logging formats as processor and verifier
- handle error, 4xx, 5xx status and log them
- move logging inside webhook module, away from core.clj

also refactored weepinbell.utils.digest. business logic no longer depend
on a concrete algorithm like SHA-256 or MD5, it's just a `digest`
function whose defaults can be replaced anytime.

use Java introp instead of cljc-md5, which is heavy on dependencies.