Add config option to make create-on-push repositories public by default

[TTWNO]

This is a super simple config addition to allow public-by-default push-to-create repositories.

The option DEFAULT_PUSH_CREATE_PRIVATE is true by default, but can be overridden in the config file.
If there is anything I missed, let me know.

[codecov-commenter]

Codecov Report

Merging #12936 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #12936   +/-   ##
=======================================
  Coverage   42.84%   42.85%           
=======================================
  Files         662      662           
  Lines       73002    73002           
=======================================
+ Hits        31279    31282    +3     
+ Misses      36652    36650    -2     
+ Partials     5071     5070    -1     

Impacted Files	Coverage Δ
modules/setting/repository.go	56.36% <ø> (ø)
services/repository/repository.go	48.88% <100.00%> (ø)
modules/avatar/avatar.go	50.00% <0.00%> (-4.77%)	⬇️
services/pull/pull.go	41.27% <0.00%> (+0.49%)	⬆️
modules/util/timer.go	85.71% <0.00%> (+42.85%)	⬆️

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[a1012112796]

@TTWNO @lunny I have added chinese translation in TTWNO#1, Please have a look, Thanks

[techknowlogick]

Thanks for PR :)

[jolheiser]

This also needs an entry in https://github.com/go-gitea/gitea/blob/master/custom/conf/app.example.ini