New Issue ?body= query

[KatieFrogs]

Like with mailto email links, Github allows filling the body of the issue with the "body" query string in the URL, you can see it here:
https://github.com/LoveEevee/gitea/issues/new?body=This+is+an+example
This is particularly useful to make a link in an application and include a bug reporting button, which pre-fills the issue with a specific template, but also with some debug information that the software author wants to be included with the bug.
This PR implements an optional "body" that may be included with the URL when creating a new issue in a repository.

[codecov-io]

Codecov Report

Merging #5851 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5851      +/-   ##
==========================================
+ Coverage   37.83%   37.85%   +0.01%     
==========================================
  Files         328      328              
  Lines       48326    48328       +2     
==========================================
+ Hits        18285    18294       +9     
+ Misses      27408    27401       -7     
  Partials     2633     2633

Impacted Files	Coverage Δ
routers/repo/issue.go	36.69% <100%> (+0.11%)	⬆️
models/repo_list.go	63.29% <0%> (-1.27%)	⬇️
routers/repo/view.go	47.3% <0%> (+1.19%)	⬆️
modules/process/manager.go	81.15% <0%> (+4.34%)	⬆️
models/unit.go	14.28% <0%> (+14.28%)	⬆️

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[kolaente]

Can we put this in the docs?
I'd propose to put a sentence about this in https://docs.gitea.io/en-us/issue-pull-request-templates/

[KatieFrogs]

@kolaente I have updated the documentation, thanks for the suggestion.

[kolaente]

Doesn't this also work for pull requests? If so, you should mention that 🙂

[KatieFrogs]

It does not, this PR adds supports only for new issues. Implementing this to new pull requests makes little sense to me, as the developer also has to know which branches the user needs to compare in advance.

[kolaente]

Makes sense, thanks for clarifying!