Fix repository create/delete event webhooks

[jamesorlakin]

This small PR changes the webhook trigger behaviour to be more in line with what's expected. (When 'repository' events are enabled, of course)

In other words:

• For system-wide or default webhooks, repository events will now trigger said webhook. Previously it had to be under an organization for create events to be visible - a tad unexpected!
• Deleting a repository will now fire its own defined webhooks, not just organisational and system ones.

In order to enable the latter the webhook has to now be triggered before the actual repo undergoes deletion. I'm willing to tweak this to try and 'grab' the webhook model beforehand and trigger the webhook notifier directly afterwards, but this may make the code more complex for little benefit.

Closes #11766, #9180.

[codecov-commenter]

Codecov Report

Merging #13008 into master will increase coverage by 0.03%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master   #13008      +/-   ##
==========================================
+ Coverage   42.58%   42.62%   +0.03%     
==========================================
  Files         671      671              
  Lines       73632    73629       -3     
==========================================
+ Hits        31358    31381      +23     
+ Misses      37188    37168      -20     
+ Partials     5086     5080       -6     

Impacted Files	Coverage Δ
modules/notification/webhook/webhook.go	49.30% <71.42%> (+0.86%)	⬆️
services/repository/repository.go	52.27% <100.00%> (+3.38%)	⬆️
modules/log/event.go	57.54% <0.00%> (-1.89%)	⬇️
models/gpg_key.go	53.33% <0.00%> (-0.58%)	⬇️
models/error.go	34.85% <0.00%> (+0.50%)	⬆️
modules/git/repo.go	46.70% <0.00%> (+0.50%)	⬆️
services/pull/check.go	48.46% <0.00%> (+0.76%)	⬆️
modules/indexer/issues/indexer.go	60.00% <0.00%> (+1.53%)	⬆️
models/unit.go	49.31% <0.00%> (+2.73%)	⬆️
modules/git/utils.go	77.04% <0.00%> (+3.27%)	⬆️
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 77f3dbe...ab349e9. Read the comment docs.

[jamesorlakin]

Does this want a backport?