Comment box tweaks and SVG dropdown triangles (#13376)

* Comment box tweaks and SVG dropdown triangles

- Change all dropdown triangles to SVG
- Bring inline review comment box closer to regular comment boxes
- Enhance arc-green checkbox contrast
- Minor reaction tweaks
- Flexbox the diff file header

* remove a border

* fix type marker in arc-green

* add small code padding

* fix position regression and remove useless rules

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

templates/admin/auth/source/oauth.tmpl

@@ -4,7 +4,7 @@
 		<div class="ui selection type dropdown">
 			<input type="hidden" id="oauth2_provider" name="oauth2_provider" value="{{.oauth2_provider}}">
 			<div class="text">{{.oauth2_provider}}</div>
-			<i class="dropdown icon"></i>
+			{{svg "octicon-triangle-down" 14 "dropdown icon"}}
 			<div class="menu">
 				{{range $key, $value := .OAuth2Providers}}
 					<div class="item" data-value="{{$key}}">{{$value.DisplayName}}</div>