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

models/issues/comment.go

@@ -24,6 +24,7 @@ import (
 	"code.gitea.io/gitea/modules/htmlutil"
 	"code.gitea.io/gitea/modules/json"
 	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/markup"
 	"code.gitea.io/gitea/modules/optional"
 	"code.gitea.io/gitea/modules/references"
 	"code.gitea.io/gitea/modules/structs"
@@ -543,6 +544,12 @@ func (c *Comment) EventTag() string {
 	return fmt.Sprintf("event-%d", c.ID)
 }
 
+func (c *Comment) GetSanitizedContentHTML() template.HTML {
+	// mainly for type=4 CommentTypeCommitRef
+	// the content is a link like <a href="{RepoLink}/commit/{CommitID}">message title</a> (from CreateRefComment)
+	return markup.Sanitize(c.Content)
+}
+
 // LoadLabel if comment.Type is CommentTypeLabel, then load Label
 func (c *Comment) LoadLabel(ctx context.Context) error {
 	var label Label