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/view_content/pull.tmpl

@@ -336,7 +336,7 @@
 									</span>
 								</button>
 								<div class="ui dropdown icon button">
-									<i class="dropdown icon"></i>
+									{{svg "octicon-triangle-down" 14 "dropdown icon"}}
 									<div class="menu">
 										{{if $prUnit.PullRequestsConfig.AllowMerge}}
 										<div class="item{{if eq .MergeStyle "merge"}} active selected{{end}}" data-do="merge">{{$.i18n.Tr "repo.pulls.merge_pull_request"}}</div>