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

web_src/less/_repository.less

@@ -106,10 +106,6 @@
         margin-right: 8.75px;
       }
 
-      .hide {
-        display: none !important;
-      }
-
       .dependency {
         padding: 0;
         white-space: nowrap;
@@ -2592,18 +2588,10 @@
   }
 }
 
-#issue-filters.hide {
-  display: none;
-}
-
 #issue-actions {
   margin-top: -1rem !important; // counteract padding from Semantic
 }
 
-#issue-actions.hide {
-  display: none;
-}
-
 .ui.menu .item > img:not(.ui) {
   width: auto;
 }