Fix margin on the new/edit milestone page (#25801)

There is some distortion in desktop and mobile ui for new/edit milestone
page.
Fixing the new/edit milestone page for desktop and mobile ui

Design background
https://uxplanet.org/primary-secondary-action-buttons-c16df9b36150
https://balsamiq.com/learn/articles/button-design-best-practices/


<details>
  <summary>Screen shots</summary>
  
Before:

![image](https://github.com/go-gitea/gitea/assets/80308335/c8627679-da2d-465f-bc8f-978d7b836919)

![image](https://github.com/go-gitea/gitea/assets/80308335/43b54599-2856-440a-a13c-08bb3cedf35c)

After

![image](https://github.com/go-gitea/gitea/assets/80308335/85ca3d32-4842-41bc-8647-aa4222b9cc06)

![image](https://github.com/go-gitea/gitea/assets/80308335/c00e87c4-e64e-4b18-aa42-d26c3942a21b)

</details>

---------

Co-authored-by: Denys Konovalov <privat@denyskon.de>
Co-authored-by: Giteabot <teabot@gitea.io>

templates/repo/issue/milestone_new.tmpl

@@ -21,9 +21,8 @@
 			{{end}}
 		</h2>
 		{{template "base/alert" .}}
-		<form class="ui form grid" action="{{.Link}}" method="post">
+		<form class="ui form" action="{{.Link}}" method="post">
 			{{.CsrfTokenHtml}}
-			<div class="twelve wide column">
 				<div class="field {{if .Err_Title}}error{{end}}">
 					<label>{{.locale.Tr "repo.milestones.title"}}</label>
 					<input name="title" placeholder="{{.locale.Tr "repo.milestones.title"}}" value="{{.title}}" autofocus required maxlength="50">
@@ -39,10 +38,8 @@
 					<label>{{.locale.Tr "repo.milestones.desc"}}</label>
 					<textarea name="content">{{.content}}</textarea>
 				</div>
-			</div>
-			<div class="ui container">
 				<div class="divider"></div>
-				<div class="ui right">
+				<div class="gt-text-right">
 					{{if .PageIsEditMilestone}}
 						<a class="ui primary basic button" href="{{.RepoLink}}/milestones">
 							{{.locale.Tr "repo.milestones.cancel"}}
@@ -56,7 +53,6 @@
 						</button>
 					{{end}}
 			</div>
-			</div>
 		</form>
 	</div>
 </div>