Improve flex list UI (#26970)

1. There is already `gt-ac`, so no need to introduce `flex-item-center`
2. The `flex-item-baseline` and `.flex-item-icon svg { margin-top: 1px
}` seem to be a tricky patch, they don't resolve the root problem, and
still cause misalignment in some cases.
* The root problem is: the "icon" needs to align with the sibling
"title"
    * So, make the "icon" and the "title" both have the same height 
3. `flex-text-inline` could only be used if the element is really
"inline", otherwise its `vertical-align` would make the box size change.
In most cases, `flex-text-block` is good enough.


![image](https://github.com/go-gitea/gitea/assets/2114189/1b7acfc2-b1c7-4e9c-a983-2fa932026479)

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>

templates/shared/issuelist.tmpl

@@ -1,10 +1,12 @@
 <div id="issue-list" class="flex-list">
 	{{$approvalCounts := .ApprovalCounts}}
 	{{range .Issues}}
-		<div class="flex-item flex-item-baseline">
-			<div class="flex-item-leading gt-ac">
+		<div class="flex-item">
+			<div class="flex-item-leading">
 				{{if $.CanWriteIssuesOrPulls}}
+				<div class="flex-item-icon">
 					<input type="checkbox" autocomplete="off" class="issue-checkbox gt-mr-4" data-issue-id={{.ID}} aria-label="{{$.locale.Tr "repo.issues.action_check"}} &quot;{{.Title}}&quot;">
+				</div>
 				{{end}}
 				<div class="flex-item-icon">
 					{{template "shared/issueicon" .}}
@@ -44,7 +46,7 @@
 						{{end}}
 						{{if .NumComments}}
 						<div class="text grey">
-							<a class="gt-no-underline muted flex-text-inline" href="{{if .Link}}{{.Link}}{{else}}{{$.Link}}/{{.Index}}{{end}}">
+							<a class="gt-no-underline muted flex-text-block" href="{{if .Link}}{{.Link}}{{else}}{{$.Link}}/{{.Index}}{{end}}">
 								{{svg "octicon-comment" 16}}{{.NumComments}}
 							</a>
 						</div>

templates/shared/secrets/add_list.tmpl

@@ -14,7 +14,7 @@
 	{{if .Secrets}}
 	<div class="flex-list">
 		{{range .Secrets}}
-		<div class="flex-item flex-item-center">
+		<div class="flex-item gt-ac">
 			<div class="flex-item-leading">
 				{{svg "octicon-key" 32}}
 			</div>

templates/shared/variables/variable_list.tmpl

@@ -16,7 +16,7 @@
 	{{if .Variables}}
 	<div class="flex-list">
 		{{range .Variables}}
-		<div class="flex-item flex-item-center">
+		<div class="flex-item gt-ac">
 			<div class="flex-item-leading">
 				{{svg "octicon-pencil" 32}}
 			</div>