- (send-webhook) does not need to validate URLs; (trigger) already does
that; replace validation logic with :pre conditional
- (make-request) does not need to fallback to empty string when
event-key is nil. it should never be nil.

- processor will call hook functions with webmention data
- refactor webhook.clj massively
- add dynamic parameter substitution for urls, e.g. {source} {target}
{event}......
- add config option :webhook-request, a function with parameters [hook,
data] that returns a map (resembling {:headers ... :body ...}).
webhook.clj uses this function to generate payload.
- allow map besids URL string in config option :webhook. if it's a URL,
sends webhook request to it with payload generated by :webhook-request;
if it's a map, it should be like {:url ... :request-fn ...}, which
specifies a payload generator.
- add request-fn preset :ntfy

starting from now, every commit should be cljfmt-fixed

comment

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

used java.net to create a small validator that passes the test

refactor: rename validator to url, and move other url funcs here