Refactor some Str2html code (#29397)

This PR touches the most interesting part of the "template refactoring".

1. Unclear variable type. Especially for "web/feed/convert.go":
sometimes it uses text, sometimes it uses HTML.
2. Assign text content to "RenderedContent" field, for example: `
project.RenderedContent = project.Description` in web/org/projects.go
3. Assign rendered content to text field, for example: `r.Note =
rendered content` in web/repo/release.go
4. (possible) Incorrectly calling `{{Str2html
.PackageDescriptor.Metadata.ReleaseNotes}}` in
package/content/nuget.tmpl, I guess the name Str2html misleads
developers to use it to "render string to html", but it only sanitizes.
if ReleaseNotes really contains HTML, then this is not a problem.

models/issues/issue.go

@@ -7,6 +7,7 @@ package issues
 import (
 	"context"
 	"fmt"
+	"html/template"
 	"regexp"
 	"slices"
 
@@ -105,7 +106,7 @@ type Issue struct {
 	OriginalAuthorID int64                  `xorm:"index"`
 	Title            string                 `xorm:"name"`
 	Content          string                 `xorm:"LONGTEXT"`
-	RenderedContent  string                 `xorm:"-"`
+	RenderedContent  template.HTML          `xorm:"-"`
 	Labels           []*Label               `xorm:"-"`
 	MilestoneID      int64                  `xorm:"INDEX"`
 	Milestone        *Milestone             `xorm:"-"`