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/issue/view_content/pull_merge_instruction.tmpl

@@ -1,6 +1,6 @@
 <div class="ui divider"></div>
 <div class="instruct-toggle"> {{$.locale.Tr "repo.pulls.merge_instruction_hint" | Safe}} </div>
-<div class="instruct-content gt-mt-3" style="display:none">
+<div class="instruct-content gt-mt-3 gt-hidden">
 	<div><h3 class="gt-di">{{$.locale.Tr "step1"}} </h3>{{$.locale.Tr "repo.pulls.merge_instruction_step1_desc"}}</div>
 	<div class="ui secondary segment">
 		{{if eq $.Issue.PullRequest.Flow 0}}