calculating hash using old unnormalized URLs will most likely produce
the same old invalid hash, this commit normalizes URLs first and then
calculate new hash, updating old URLs as well.

- (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.

- simplify (.mkdirs parent-dir) and with (io/make-parents file)
- should return false when HTTP request fails, not nil
- check if corrputed file exitsts before deleting it
- add tests

closes #75

This is a large squash commit. For details, see https://codeberg.org/eltrac/weepinbell/pulls/80

- use LEFT JOIN instead of making multiple queries
- put insert/update inside transactions
- add indexes
- fix configuration description