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/milestone.go

@@ -6,6 +6,7 @@ package issues
 import (
 	"context"
 	"fmt"
+	"html/template"
 	"strings"
 
 	"code.gitea.io/gitea/models/db"
@@ -47,8 +48,8 @@ type Milestone struct {
 	RepoID          int64                  `xorm:"INDEX"`
 	Repo            *repo_model.Repository `xorm:"-"`
 	Name            string
-	Content         string `xorm:"TEXT"`
-	RenderedContent string `xorm:"-"`
+	Content         string        `xorm:"TEXT"`
+	RenderedContent template.HTML `xorm:"-"`
 	IsClosed        bool
 	NumIssues       int
 	NumClosedIssues int