UI: Repo header tweaks (#5945)

* UI: Repo header tweaks

- Use basic buttons on the header buttons, matching gogs
- Make 'Manage topic' text smaller, remove margin-left with no topics
present
- Move various inline styles to CSS
- Use flexbox on header title and buttons

* fix indentation

* reverse media query wrapping

* fix inconsisten whitespace

public/less/_base.less

@@ -12,6 +12,9 @@ img {
 table {
     border-collapse: collapse;
 }
+a {
+    cursor: pointer;
+}
 .rounded {
     border-radius: .28571429rem !important;
 }