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

modules/markup/html.go

@@ -6,6 +6,7 @@ package markup
 import (
 	"bytes"
 	"fmt"
+	"html/template"
 	"io"
 	"regexp"
 	"slices"
@@ -149,9 +150,9 @@ func PostProcessDefault(ctx *RenderContext, input io.Reader, output io.Writer) e
 	return postProcess(ctx, procs, input, output)
 }
 
-// PostProcessCommitMessage will use the same logic as PostProcess, but will disable
-// the shortLinkProcessor.
-func PostProcessCommitMessage(ctx *RenderContext, content string) (string, error) {
+// PostProcessCommitMessage will use the same logic as PostProcess, but will disable the shortLinkProcessor.
+// FIXME: this function and its family have a very strange design: it takes HTML as input and output, processes the "escaped" content.
+func PostProcessCommitMessage(ctx *RenderContext, content template.HTML) (template.HTML, error) {
 	procs := []processor{
 		fullIssuePatternProcessor,
 		comparePatternProcessor,
@@ -165,7 +166,8 @@ func PostProcessCommitMessage(ctx *RenderContext, content string) (string, error
 		emojiProcessor,
 		emojiShortCodeProcessor,
 	}
-	return postProcessString(ctx, procs, content)
+	s, err := postProcessString(ctx, procs, string(content))
+	return template.HTML(s), err
 }
 
 var emojiProcessors = []processor{