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/repo/issue/branch_selector_field.tmpl

@@ -8,7 +8,7 @@
 <div class="ui {{if .ReadOnly}}disabled{{end}} floating filter select-branch dropdown" data-no-results="{{.i18n.Tr "repo.pulls.no_results"}}">
 	<div class="ui basic small button">
 		<span class="text branch-name">{{if .Issue.Ref}}{{$.RefEndName}}{{else}}{{.i18n.Tr "repo.issues.no_ref"}}{{end}}</span>
-		<i class="dropdown icon"></i>
+		{{svg "octicon-triangle-down" 14 "dropdown icon"}}
 	</div>
 	<div class="menu">
 		<div class="ui icon search input">