* Like #22851
* All other dropdown menu elements do not have such `hidden` class.
* Actually the dropdown menu elements do not need it in HTML, so this PR
removes it.
* There is already `.ui.dropdown .menu { display: none; }`, so when
loading the page, the menu is correctly hidden initially, no need to add
any more CSS classes.
* The Fomantic UI's `.hidden` class should still exist until there is no
its checkbox/dropdown module anymore. The Fomantic UI JS code still
addes `hidden` for `ui checkbox` and addes `transition hidden` for `ui
menu` at the moment.
* This PR also cleans the legacy inline `style`, which is quite hacky
and no need anymore.
All these dropdown menus work well.
I have tested these 5 places:
* The code search from User Profile
* The issue sidebar to lock issue
* The repo search form
* The repo setting page: branch list
* The repo setting page: merge option list
Screenshot:
d306ab2b8f
