read-only checkboxes don't appear and don't entirely act the way one might expect (#25573) (#25602)

Backport #25573 by @saegl5

This pull request fades read-only checkboxes and checkmark, and it makes
the checkboxes act more read-only/disabled by not changing the
border-color when clicked.

Examples using light mode:
 
| Before | After |
| - | - |
| ![Kapture 2023-06-28 at 00 20
45](https://github.com/go-gitea/gitea/assets/63764270/0899fd5c-18a9-4290-9ba9-d3cf71033cf8)
| ![Kapture 2023-06-28 at 00 23
12](https://github.com/go-gitea/gitea/assets/63764270/0db9be14-e16c-42ed-8fb1-999928fd1d25)
|
| ![Kapture 2023-06-28 at 00 25
22](https://github.com/go-gitea/gitea/assets/63764270/65c6c380-b928-4e6c-b403-3655d3565896)
| ![Kapture 2023-06-28 at 00 27
28](https://github.com/go-gitea/gitea/assets/63764270/d8c2a019-e07c-43a1-a7fa-93c0d4e01900)
|
| | read-only checkboxes and checkmark are faded<br>and the checkboxes
act more read-only/disabled |

Fixes/Closes/Resolves #25076

Co-authored-by: Ed Silkworth <ed.silkworth@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/pulls/fork.tmpl

@@ -45,8 +45,8 @@
 					</div>
 					<div class="inline field">
 						<label>{{.locale.Tr "repo.visibility"}}</label>
-						<div class="ui read-only checkbox">
-							<input type="checkbox" {{if .IsPrivate}}checked{{end}}>
+						<div class="ui disabled checkbox">
+							<input type="checkbox" disabled {{if .IsPrivate}}checked{{end}}>
 							<label>{{.locale.Tr "repo.visibility_helper" | Safe}}</label>
 						</div>
 						<span class="help">{{.locale.Tr "repo.fork_visibility_helper"}}</span>