Misc UI fixes, add secondary color (#13378)

* Misc UI fixes, add secondary color

- Add secondary color, primarily used in arc-green currently
- Convert icons on release page to SVG
- Improve resolved conversation placeholder
- Diff fixes on arc-green
- Misc color tweaks

* fix comment header, adjust arc-green dropzone

* label margin, sidebar margin

* flexbox commits table and add primary button styles

* tooltip styles

* file header fixes

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: zeripath <art27@cantab.net>

templates/repo/issue/view_content/comments.tmpl

@@ -19,15 +19,15 @@
 			</a>
 		{{end}}
 			<div class="content">
-				<div class="ui top attached header">
-					<div class="header-left df ac">
+				<div class="ui top attached header comment-header df ac sb">
+					<div class="comment-header-left df ac">
 						{{if .OriginalAuthor }}
 							<span class="text black"><i class="fa {{MigrationIcon $.Repository.GetOriginalURLHostname}}" aria-hidden="true"></i> {{ .OriginalAuthor }}</span><span class="text grey"> {{$.i18n.Tr "repo.issues.commented_at" .Issue.HashTag $createdStr | Safe}} {{if $.Repository.OriginalURL}}</span><span class="text migrate">({{$.i18n.Tr "repo.migrated_from" $.Repository.OriginalURL $.Repository.GetOriginalURLHostname | Safe }}){{end}}</span>
 						{{else}}
 							<span class="text grey"><a class="author"{{if gt .Poster.ID 0}} href="{{.Poster.HomeLink}}"{{end}}>{{.Poster.GetDisplayName}}</a> {{$.i18n.Tr "repo.issues.commented_at" .HashTag $createdStr | Safe}}</span>
 						{{end}}
 					</div>
-					<div class="header-right actions df ac">
+					<div class="comment-header-right actions df ac">
 						{{if not $.Repository.IsArchived}}
 							{{if eq .PosterID .Issue.PosterID }}
 								<div class="ui basic label">