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/repo/release.go

@@ -7,6 +7,7 @@ package repo
 import (
 	"context"
 	"fmt"
+	"html/template"
 	"net/url"
 	"sort"
 	"strconv"
@@ -80,7 +81,7 @@ type Release struct {
 	NumCommits       int64
 	NumCommitsBehind int64              `xorm:"-"`
 	Note             string             `xorm:"TEXT"`
-	RenderedNote     string             `xorm:"-"`
+	RenderedNote     template.HTML      `xorm:"-"`
 	IsDraft          bool               `xorm:"NOT NULL DEFAULT false"`
 	IsPrerelease     bool               `xorm:"NOT NULL DEFAULT false"`
 	IsTag            bool               `xorm:"NOT NULL DEFAULT false"` // will be true only if the record is a tag and has no related releases