Remove Fomantic-UI's `.hidden` CSS class for menu elements (#22895)

* Like #22851
* All other dropdown menu elements do not have such `hidden` class.
* Actually the dropdown menu elements do not need it in HTML, so this PR
removes it.
* There is already `.ui.dropdown .menu { display: none; }`, so when
loading the page, the menu is correctly hidden initially, no need to add
any more CSS classes.
* The Fomantic UI's `.hidden` class should still exist until there is no
its checkbox/dropdown module anymore. The Fomantic UI JS code still
addes `hidden` for `ui checkbox` and addes `transition hidden` for `ui
menu` at the moment.
* This PR also cleans the legacy inline `style`, which is quite hacky
and no need anymore.

All these dropdown menus work well.

I have tested these 5 places:

* The code search from User Profile
* The issue sidebar to lock issue
* The repo search form
* The repo setting page: branch list
* The repo setting page: merge option list

Screenshot:

![image](https://user-images.githubusercontent.com/2114189/218534515-e7dfe291-6765-4e0b-833c-b74f17e30117.png)

templates/code/searchform.tmpl

@@ -4,7 +4,7 @@
 		<div class="ui dropdown selection tooltip{{if .CodeIndexerUnavailable}} disabled{{end}}" data-content="{{.locale.Tr "explore.search.type.tooltip"}}">
 			<input name="t" type="hidden" value="{{.queryType}}"{{if .CodeIndexerUnavailable}} disabled{{end}}>{{svg "octicon-triangle-down" 14 "dropdown icon"}}
 			<div class="text">{{.locale.Tr (printf "explore.search.%s" (or .queryType "fuzzy"))}}</div>
-			<div class="menu transition hidden" tabindex="-1" style="display: block !important;">
+			<div class="menu">
 				<div class="item tooltip" data-value="" data-content="{{.locale.Tr "explore.search.fuzzy.tooltip"}}">{{.locale.Tr "explore.search.fuzzy"}}</div>
 				<div class="item tooltip" data-value="match" data-content="{{.locale.Tr "explore.search.match.tooltip"}}">{{.locale.Tr "explore.search.match"}}</div>
 			</div>

templates/repo/issue/view_content/sidebar.tmpl

@@ -639,7 +639,7 @@
 
 									<div class="default text"> </div>
 
-									<div class="menu transition hidden" tabindex="-1" style="display: block !important;">
+									<div class="menu">
 										{{range .LockReasons}}
 											<div class="item" data-value="{{.}}">{{.}}</div>
 										{{end}}

templates/repo/search.tmpl

@@ -9,7 +9,7 @@
 					<div class="ui dropdown selection tooltip{{if .CodeIndexerUnavailable}} disabled{{end}}" data-content="{{.locale.Tr "repo.search.type.tooltip"}}">
 						<input name="t" type="hidden"{{if .CodeIndexerUnavailable}} disabled{{end}} value="{{.queryType}}">{{svg "octicon-triangle-down" 14 "dropdown icon"}}
 						<div class="text">{{.locale.Tr (printf "repo.search.%s" (or .queryType "fuzzy"))}}</div>
-						<div class="menu transition hidden" tabindex="-1" style="display: block !important;">
+						<div class="menu">
 							<div class="item tooltip" data-value="" data-content="{{.locale.Tr "repo.search.fuzzy.tooltip"}}">{{.locale.Tr "repo.search.fuzzy"}}</div>
 							<div class="item tooltip" data-value="match" data-content="{{.locale.Tr "repo.search.match.tooltip"}}">{{.locale.Tr "repo.search.match"}}</div>
 						</div>

templates/repo/settings/branches.tmpl

@@ -29,7 +29,7 @@
 								{{end}}
 							</select>{{svg "octicon-triangle-down" 14 "dropdown icon"}}
 							<div class="default text">{{.Repository.DefaultBranch}}</div>
-							<div class="menu transition hidden" tabindex="-1" style="display: block !important;">
+							<div class="menu">
 								{{range .Branches}}
 									<div class="item" data-value="{{.}}">{{.}}</div>
 								{{end}}

templates/repo/settings/options.tmpl

@@ -538,7 +538,7 @@
 										{{.locale.Tr "repo.pulls.squash_merge_pull_request"}}
 									{{end}}
 								</div>
-								<div class="menu transition hidden" tabindex="-1" style="display: block !important;">
+								<div class="menu">
 									<div class="item" data-value="merge">{{.locale.Tr "repo.pulls.merge_pull_request"}}</div>
 									<div class="item" data-value="rebase">{{.locale.Tr "repo.pulls.rebase_merge_pull_request"}}</div>
 									<div class="item" data-value="rebase-merge">{{.locale.Tr "repo.pulls.rebase_merge_commit_pull_request"}}</div>