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/migrate/github.tmpl

@@ -85,7 +85,7 @@
 								<img class="ui mini image" src="{{.ContextUser.RelAvatarLink}}">
 								{{.ContextUser.ShortName 20}}
 							</span>
-							<i class="dropdown icon"></i>
+							{{svg "octicon-triangle-down" 14 "dropdown icon"}}
 							<div class="menu" title="{{.SignedUser.Name}}">
 								<div class="item" data-value="{{.SignedUser.ID}}">
 									<img class="ui mini image" src="{{.SignedUser.RelAvatarLink}}">