Refactor hiding-methods, remove jQuery show/hide, remove .hide class, remove inline style=display:none (#22950)

Close #22847

This PR:

* introduce Gitea's own `showElem` and related functions
* remove jQuery show/hide
* remove .hide class
* remove inline style=display:none 

From now on:

do not use:
* "[hidden]" attribute: it's too weak, can not be applied to an element
with "display: flex"
* ".hidden" class: it has been polluted by Fomantic UI in many cases
* inline style="display: none": it's difficult to tweak
* jQuery's show/hide/toggle: it can not show/hide elements with
"display: xxx !important"

only use:
* this ".gt-hidden" class
* showElem/hideElem/toggleElem functions in "utils/dom.js"

cc: @silverwind , this is the all-in-one PR

templates/repo/commits_list.tmpl

@@ -72,7 +72,7 @@
 							{{end}}
 							{{template "repo/commit_statuses" dict "Status" .Status "Statuses" .Statuses "root" $}}
 							{{if IsMultilineCommitMessage .Message}}
-							<pre class="commit-body" style="display: none;">{{RenderCommitBody $.Context .Message $commitRepoLink $.Repository.ComposeMetas}}</pre>
+							<pre class="commit-body gt-hidden">{{RenderCommitBody $.Context .Message $commitRepoLink $.Repository.ComposeMetas}}</pre>
 							{{end}}
 						</td>
 						{{if .Committer}}