css: make plugin button container a flexbox

backend: pass plugin button generated code through domdocument to ensure
its correctness; set data-plugin-name attribute on children to make
them sortable via css

themes/light/tt-rss.less

@@ -22,6 +22,7 @@ body.ttrss_main {
 
 			.left, .right {
 				display : flex;
+				align-items : center;
 			}
 
 			.row {