Prevent simultaneous editing of comments and issues (#31053)

fixes #22907

Tested:
- [x] issue content edit
- [x] issue content change tasklist
- [x] pull request content edit
- [x] pull request change tasklist

![issue-content-edit](https://github.com/go-gitea/gitea/assets/29250154/a0828889-fb96-4bc4-8600-da92e3205812)

(cherry picked from commit aa92b13164e84c26be91153b6022220ce0a27720)

Conflicts:
	models/issues/comment.go
	 c7a389f2b2 [FEAT] allow setting the update date on issues and comments

	options/locale/locale_en-US.ini
	 trivial context conflicts

	routers/api/v1/repo/issue_comment.go
	routers/api/v1/repo/issue_comment_attachment.go
	services/issue/comments.go
	services/issue/content.go
         user blocking is implemented differently in Forgejo

	routers/web/repo/issue.go
	 trivial difference from 6a0750177f Allow to save empty comment
         user blocking is implemented differently in Forgejo

	templates/repo/issue/view_content/conversation.tmpl
	 templates changed a lot in Forgejo but the change is
	 trivially ported

	tests/integration/issue_test.go
	 other tests were added in the same region

	web_src/js/features/repo-issue-edit.js
	 the code is still web_src/js/features/repo-legacy.js
	 trivially ported

models/issues/issue.go

@@ -94,6 +94,8 @@ func (err ErrIssueWasClosed) Error() string {
 	return fmt.Sprintf("Issue [%d] %d was already closed", err.ID, err.Index)
 }
 
+var ErrIssueAlreadyChanged = util.NewInvalidArgumentErrorf("the issue is already changed")
+
 // Issue represents an issue or pull request of repository.
 type Issue struct {
 	ID               int64                  `xorm:"pk autoincr"`
@@ -107,6 +109,7 @@ type Issue struct {
 	Title            string                 `xorm:"name"`
 	Content          string                 `xorm:"LONGTEXT"`
 	RenderedContent  template.HTML          `xorm:"-"`
+	ContentVersion   int                    `xorm:"NOT NULL DEFAULT 0"`
 	Labels           []*Label               `xorm:"-"`
 	MilestoneID      int64                  `xorm:"INDEX"`
 	Milestone        *Milestone             `xorm:"-"`