Move webhook into models/webhook/

[lunny]

As title

[codecov-commenter]

Codecov Report

Merging #17579 (8081801) into main (30515f2) will increase coverage by 0.03%.
The diff coverage is 47.47%.

@@            Coverage Diff             @@
##             main   #17579      +/-   ##
==========================================
+ Coverage   45.48%   45.52%   +0.03%     
==========================================
  Files         793      795       +2     
  Lines       88846    88855       +9     
==========================================
+ Hits        40416    40449      +33     
+ Misses      41918    41896      -22     
+ Partials     6512     6510       -2     

Impacted Files	Coverage Δ
models/db/error.go	0.00% <0.00%> (ø)
models/error.go	39.86% <ø> (+0.35%)	⬆️
models/issue_dependency.go	63.76% <0.00%> (ø)
models/lfs_lock.go	57.14% <0.00%> (ø)
models/notification.go	66.15% <0.00%> (ø)
models/repo_avatar.go	35.78% <0.00%> (ø)
models/repo_collaboration.go	57.89% <0.00%> (ø)
models/repo_generate.go	15.00% <0.00%> (ø)
models/statistic.go	0.00% <0.00%> (ø)
modules/cron/tasks.go	48.78% <0.00%> (ø)
... and 101 more

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