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/org/member/members.tmpl

@@ -7,7 +7,7 @@
 		<div class="flex-list">
 			{{range .Members}}
 				{{$isPublic := index $.MembersIsPublicMember .ID}}
-				<div class="flex-item {{if $.PublicOnly}}flex-item-center{{end}}">
+				<div class="flex-item {{if $.PublicOnly}}gt-ac{{end}}">
 					<div class="flex-item-leading">
 						<a href="{{.HomeLink}}">{{ctx.AvatarUtils.Avatar . 48}}</a>
 					</div>

templates/org/team/members.tmpl

@@ -24,7 +24,7 @@
 				<div class="ui attached segment">
 					<div class="flex-list">
 						{{range .Team.Members}}
-							<div class="flex-item flex-item-center">
+							<div class="flex-item gt-ac">
 								<div class="flex-item-leading">
 									<a href="{{.HomeLink}}">{{ctx.AvatarUtils.Avatar . 32}}</a>
 								</div>
@@ -56,7 +56,7 @@
 				<div class="ui attached segment">
 					<div class="flex-list">
 						{{range .Invites}}
-							<div class="flex-item flex-item-center">
+							<div class="flex-item gt-ac">
 								<div class="flex-item-main">
 									{{.Email}}
 								</div>

templates/org/team/repositories.tmpl

@@ -28,7 +28,7 @@
 				<div class="ui attached segment">
 					<div class="flex-list">
 						{{range .Team.Repos}}
-							<div class="flex-item flex-item-center">
+							<div class="flex-item gt-ac">
 								<div class="flex-item-leading">
 									{{template "repo/icon" .}}
 								</div>