Refactor flash message and remove SanitizeHTML template func (#37179)

1. Fix the "flash message" layout problem for different cases
* I am sure most of the users should have ever seen the ugly
center-aligned error message with multiple lines.
2. Fix inconsistent "Details" flash message EOL handling, sometimes
`\n`, sometimes `<br>`
   * Now, always use "\n" and use `<pre>` to render
3. Remove SanitizeHTML template func because it is not useful and can be
easily abused.
* But it is still kept for mail templates, for example:
https://github.com/go-gitea/gitea/issues/36049
4. Clarify PostProcessCommitMessage's behavior and add FIXME comment

By the way: cleaned up some devtest pages, move embedded style block to
CSS file

routers/web/feed/convert.go

@@ -15,6 +15,7 @@ import (
 	activities_model "code.gitea.io/gitea/models/activities"
 	"code.gitea.io/gitea/models/renderhelper"
 	repo_model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/modules/markup"
 	"code.gitea.io/gitea/modules/markup/markdown"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/templates"
@@ -237,7 +238,7 @@ func feedActionsToFeedItems(ctx *context.Context, actions activities_model.Actio
 			}
 		}
 		if len(content) == 0 {
-			content = templates.SanitizeHTML(desc)
+			content = markup.Sanitize(desc)
 		}
 
 		items = append(items, &feeds.Item{