Fix new issue/pr avatar (#31419) (#31424)

Backport #31419 by @silverwind

The avatar on "New Issue" and "New Pull Request" pages was inconsistent.
Removed the extra margin and the new CSS rules now use common parent
`<form id="#new-issue">` because `.repository.new.issue` is not present
on pull request page.

Before:

<img width="181" alt="Screenshot 2024-06-19 at 13 56 17"
src="https://github.com/go-gitea/gitea/assets/115237/5270d352-db5b-45b3-9d06-4790c17ae9b4">
<img width="213" alt="Screenshot 2024-06-19 at 13 54 02"
src="https://github.com/go-gitea/gitea/assets/115237/012f5607-aef0-4f48-90e3-8d4022480203">


After:

<img width="195" alt="Screenshot 2024-06-19 at 13 54 16"
src="https://github.com/go-gitea/gitea/assets/115237/e7590c66-3b28-4790-9970-33bd567eeb31">
<img width="212" alt="Screenshot 2024-06-19 at 13 54 22"
src="https://github.com/go-gitea/gitea/assets/115237/8e1cfede-614c-4cea-9af2-ada6da7a7361">

Co-authored-by: silverwind <[email protected]>

Author: GiteaBot

web_src/css/repo.css

@@ -530,14 +530,10 @@ td .commit-summary {
   min-width: 100px;
 }
 
-.repository.new.issue .comment.form .comment .avatar {
+#new-issue .avatar {
   width: 3em;
 }
 
-.repository.new.issue .comment.form .content {
-  margin-left: 4em;
-}
-
 .repository.new.issue .comment.form .content::before,
 .repository.new.issue .comment.form .content::after {
   right: 100%;