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/utils/utils.go

@@ -5,10 +5,11 @@ package utils
 
 import (
 	"html"
-	"strings"
+	"html/template"
 )
 
-// SanitizeFlashErrorString will sanitize a flash error string
-func SanitizeFlashErrorString(x string) string {
-	return strings.ReplaceAll(html.EscapeString(x), "\n", "<br>")
+// EscapeFlashErrorString will escape the flash error string
+// Maybe do more sanitization in the future, e.g.: hide sensitive information, etc.
+func EscapeFlashErrorString(x string) template.HTML {
+	return template.HTML(html.EscapeString(x))
 }