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/comment.go

@@ -8,6 +8,7 @@ package issues
 import (
 	"context"
 	"fmt"
+	"html/template"
 	"strconv"
 	"unicode/utf8"
 
@@ -259,8 +260,8 @@ type Comment struct {
 	CommitID        int64
 	Line            int64 // - previous line / + proposed line
 	TreePath        string
-	Content         string `xorm:"LONGTEXT"`
-	RenderedContent string `xorm:"-"`
+	Content         string        `xorm:"LONGTEXT"`
+	RenderedContent template.HTML `xorm:"-"`
 
 	// Path represents the 4 lines of code cemented by this comment
 	Patch       string `xorm:"-"`