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.

(cherry picked from commit e71eb8930a5d0f60874b038c223498b41ad65592)

Conflicts:
	modules/templates/util_string.go
	trivial context conflict

routers/web/repo/issue.go

@@ -42,6 +42,7 @@ import (
 	repo_module "code.gitea.io/gitea/modules/repository"
 	"code.gitea.io/gitea/modules/setting"
 	api "code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/modules/templates"
 	"code.gitea.io/gitea/modules/templates/vars"
 	"code.gitea.io/gitea/modules/timeutil"
 	"code.gitea.io/gitea/modules/util"
@@ -1769,7 +1770,7 @@ func ViewIssue(ctx *context.Context) {
 				// so "|" is used as delimeter to mark the new format
 				if comment.Content[0] != '|' {
 					// handle old time comments that have formatted text stored
-					comment.RenderedContent = comment.Content
+					comment.RenderedContent = templates.Str2html(comment.Content)
 					comment.Content = ""
 				} else {
 					// else it's just a duration in seconds to pass on to the frontend