In many cases user avatar link should be an absolute URL with http host

[wxiaoguang]

In many cases user avatar link should be an absolute URL with http host. eg: for API, for webhook.

The last PR (#17123) about user avatar refactoring make AvatarLink always returns relative URL, it is incorrect.

This PR makes AvatarLink return the absolute URL as before.

[jolheiser]

LGTM assuming tests pass

[codecov-commenter]

Codecov Report

Merging #17420 (c290d38) into main (44f2c27) will increase coverage by 0.15%.
The diff coverage is 61.96%.

@@            Coverage Diff             @@
##             main   #17420      +/-   ##
==========================================
+ Coverage   45.24%   45.40%   +0.15%     
==========================================
  Files         789      791       +2     
  Lines       88441    88674     +233     
==========================================
+ Hits        40017    40262     +245     
+ Misses      41944    41900      -44     
- Partials     6480     6512      +32     

Impacted Files	Coverage Δ
routers/api/v1/repo/wiki.go	53.51% <53.51%> (ø)
routers/api/v1/api.go	78.31% <76.92%> (-0.03%)	⬇️
models/user_avatar.go	45.67% <100.00%> (+2.08%)	⬆️
modules/convert/wiki.go	100.00% <100.00%> (ø)
models/gpg_key_common.go	59.67% <0.00%> (-4.84%)	⬇️
modules/charset/charset.go	71.71% <0.00%> (-2.03%)	⬇️
services/gitdiff/gitdiff.go	73.25% <0.00%> (-0.89%)	⬇️
models/repo_list.go	77.64% <0.00%> (-0.79%)	⬇️
models/error.go	39.51% <0.00%> (+0.48%)	⬆️
routers/api/v1/repo/pull.go	29.35% <0.00%> (+0.51%)	⬆️
... and 14 more

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