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/branch_dropdown.tmpl

@@ -27,7 +27,7 @@
 			</span>
 			{{svg "octicon-triangle-down" 14 "dropdown icon"}}
 		</button>
-		<div class="data" style="display: none" data-mode="{{if or .root.IsViewTag .isTag}}tags{{else}}branches{{end}}">
+		<div class="data gt-hidden" data-mode="{{if or .root.IsViewTag .isTag}}tags{{else}}branches{{end}}">
 			{{if $showBranchesInDropdown}}
 				{{range .root.Branches}}
 					<div class="item branch {{if eq $defaultBranch .}}selected{{end}}" data-url="{{PathEscapeSegments .}}">{{.}}</div>