Simplify helper CSS classes and avoid abuse (#26728)

Removed CSS helper classes (some of them are not useful while some of
them are abused often)

* `gt-db`: in most cases it could be replaced by `gt-df` and the flex
layout should be encouraged. Other cases: either it does need the
`gt-df` (eg: by using `div` directly) or it is an abuse (eg: the warning
message in a form)
* `gt-di`: it doesn't seem useful, or it could be replaced by `gt-dib`
in most cases.
* `gt-dif`: not useful, it could be replaced by `flex-text-inline` or
`gt-df`
* `gt-js`: never used
* All `<i class="icon gt-df gt-ac gt-jc">` could be written as `<i
class="icon">`


## Some UI samples

### Admin Notice


![image](d02010d4-dc7d-463f-bc99-dcc9b6e2e2ac)

### Admin Stacktrace


![image](4045695c-a8c4-4e37-b720-e77a61b1e965)

### Org Home


![image](069f02d0-76ad-4052-8a80-700d7e501d40)

### Org Team Repo


![image](dc8d6106-bb6b-4f60-83ac-06cb28df3ab5)

### Release List


![image](0845e8a5-d1a9-487a-9d25-3c200ad54c17)


### User Setting Application Token Scope


![image](fffbde27-432b-49c6-827e-17b8cd3457ff)

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

templates/repo/issue/labels/labels_selector_field.tmpl

@@ -8,7 +8,7 @@
 	<div class="filter menu" {{if .Issue}}data-action="update" data-issue-id="{{$.Issue.ID}}" data-update-url="{{$.RepoLink}}/issues/labels"{{else}}data-id="#label_ids"{{end}}>
 		{{if or .Labels .OrgLabels}}
 			<div class="ui icon search input">
-				<i class="icon gt-df gt-ac gt-jc">{{svg "octicon-search" 16}}</i>
+				<i class="icon">{{svg "octicon-search" 16}}</i>
 				<input type="text" placeholder="{{.locale.Tr "repo.issues.filter_labels"}}">
 			</div>
 		{{end}}