Repository transfer has to be confirmed, if user can not create repo for new owner

[6543]

close #4743

[codecov-io]

Codecov Report

Merging #14792 (5dc7c1a) into master (3d8b5ad) will increase coverage by 0.04%.
The diff coverage is 39.79%.

@@            Coverage Diff             @@
##           master   #14792      +/-   ##
==========================================
+ Coverage   42.22%   42.26%   +0.04%     
==========================================
  Files         771      774       +3     
  Lines       82181    82476     +295     
==========================================
+ Hits        34697    34858     +161     
- Misses      41832    41941     +109     
- Partials     5652     5677      +25     

Impacted Files	Coverage Δ
models/migrations/migrations.go	2.59% <ø> (ø)
models/migrations/v174.go	0.00% <0.00%> (ø)
modules/context/repo.go	53.83% <0.00%> (-1.11%)	⬇️
modules/convert/notification.go	52.27% <0.00%> (-8.26%)	⬇️
routers/repo/repo.go	28.73% <0.00%> (-2.65%)	⬇️
routers/repo/setting.go	14.37% <0.00%> (-0.53%)	⬇️
services/mailer/mail.go	61.70% <ø> (ø)
routers/repo/view.go	41.64% <25.00%> (+0.48%)	⬆️
modules/notification/mail/mail.go	38.54% <33.33%> (-0.17%)	⬇️
modules/notification/ui/ui.go	64.10% <33.33%> (-0.81%)	⬇️
... and 25 more

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[adelowo]

Thanks for wrapping this PR up @6543.