Fix install page checkboxes and dropdown width (#30526) (#30538)

Backport #30526 by @silverwind

Fixes: #30523

1. Fix checkbox rendering:

<img width="406" alt="Screenshot 2024-04-16 at 21 37 03"
src="https://github.com/go-gitea/gitea/assets/115237/42df99b0-58c7-47d1-b99d-0c15250560c7">

2. Fix width of selection dropdowns (was too small):

<img width="826" alt="Screenshot 2024-04-16 at 21 37 09"
src="https://github.com/go-gitea/gitea/assets/115237/e006d0d6-ac3d-4804-94de-b2c3c8a5900d">

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: delvh <dev.lh@web.de>

web_src/css/install.css

@@ -18,7 +18,8 @@
   width: auto;
 }
 
-.page-content.install form.ui.form input {
+.page-content.install form.ui.form input:not([type="checkbox"],[type="radio"]),
+.page-content.install form.ui.form .ui.selection.dropdown {
   width: 60%;
 }