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/diff/image_diff.tmpl

@@ -11,7 +11,7 @@
 					{{end}}
 				</div>
 			</div>
-			<div class="hide">
+			<div class="gt-hidden">
 				<div class="ui bottom attached tab image-diff-container active" data-tab="diff-side-by-side-{{.file.Index}}">
 					<div class="diff-side-by-side">
 						{{if .blobBase}}