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/labels/label_list.tmpl

@@ -6,7 +6,7 @@
 				<div class="ui dropdown type jump item">
 					<span class="text">
 						{{.i18n.Tr "repo.issues.filter_sort"}}
-						<i class="dropdown icon"></i>
+						{{svg "octicon-triangle-down" 14 "dropdown icon"}}
 					</span>
 					<div class="menu">
 						<a class="{{if or (eq .SortType "alphabetically") (not .SortType)}}active{{end}} item" href="{{$.Link}}?sort=alphabetically&state={{$.State}}">{{.i18n.Tr "repo.issues.label.filter_sort.alphabetically"}}</a>