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>

web_src/js/components/DashboardRepoList.vue

@@ -17,7 +17,7 @@
       <div class="ui attached segment repos-search">
         <div class="ui fluid right action left icon input" :class="{loading: isLoading}">
           <input type="search" spellcheck="false" maxlength="255" @input="changeReposFilter(reposFilter)" v-model="searchQuery" ref="search" @keydown="reposFilterKeyControl" :placeholder="textSearchRepos">
-          <i class="icon gt-df gt-ac gt-jc"><svg-icon name="octicon-search" :size="16"/></i>
+          <i class="icon"><svg-icon name="octicon-search" :size="16"/></i>
           <div class="ui dropdown icon button" :title="textFilter">
             <svg-icon name="octicon-filter" :size="16"/>
             <div class="menu">