Make webhook http connections resuable

[lunny]

The httplib will always create a new http.Client. That's no problem when there are few webhooks sent. But when you have hundreds of thousand webhooks to be sent, it will never reuse http connections. So let's keep only one http.Client for webhooks.

[codecov-io]

Codecov Report

Merging #6976 into master will increase coverage by 0.04%.
The diff coverage is 40%.

@@            Coverage Diff            @@
##           master   #6976      +/-   ##
=========================================
+ Coverage   41.45%   41.5%   +0.04%     
=========================================
  Files         441     440       -1     
  Lines       59699   59453     -246     
=========================================
- Hits        24747   24673      -74     
+ Misses      31727   31560     -167     
+ Partials     3225    3220       -5

Impacted Files	Coverage Δ
models/webhook.go	56.73% <40%> (-3.03%)	⬇️
models/repo_indexer.go	66.52% <0%> (-2.12%)	⬇️
models/gpg_key.go	55.83% <0%> (-0.84%)	⬇️
modules/httplib/httplib.go
modules/log/colors_router.go	87.5% <0%> (+4.16%)	⬆️

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[zeripath]

Although it appears that at present we only have a single threaded user of the webhook client in deliver it might be better to make a pool and get/release.

Otherwise looks good