Consolidate Logos and update README header (#14136)

* Consolidate Logos and update README header

- Remove unused `logo-lg.png`, `logo-sm.png` and `logo-192.png`.
- Consolidate `favicon.svg` and `logo.svg` to just `logo.svg`.
- Remove Safari Mask icon, it seems to work fine with just `favicon.png` (no SVG support).
- Remove Fluid Icon. It only served Firefox and SVG works just fine there.
- Update customization instructions.
- Update README.md to use SVG icon, increase logo size and center it and badges.

* Update README_ZH.md

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

* Update README_ZH.md

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

templates/base/head_navbar.tmpl

@@ -1,7 +1,7 @@
 <div class="ui container" id="navbar">
 	<div class="item brand" style="justify-content: space-between;">
 		<a href="{{AppSubUrl}}/">
-			<img class="ui mini image" src="{{StaticUrlPrefix}}/img/logo.svg">
+			<img class="ui mini image" with="30" height="30" src="{{StaticUrlPrefix}}/img/logo.svg">
 		</a>
 		<div class="ui basic icon button mobile-only" id="navbar-expand-toggle">
 			<i class="sidebar icon"></i>