Branch page and misc css improvements (#15208)

- Improve branches page, increase icon size, use octicons, use css vars
- Style placeholder color via css var
- Slightly increase contrast of input fields and active/hover states
- Add styling for select boxes in arc-green

web_src/less/_explore.less

@@ -48,7 +48,7 @@
 .ui.repository.branches {
   .info {
     font-size: 12px;
-    color: #808080;
+    color: var(--color-text-light);
     display: flex;
     white-space: pre;
     .commit-message {