Fix long usernames making the link difficult to see.

* Fix the width of the login box.
* Give slightly more space to the username by reducing the margin on the left side of the photo.
* set the min-height instead of the height of the header so that when usernames do take up multiple lines, the header accomodates.

BrowserID issue 985
mozilla/persona#985

static/style.less

@@ -30,7 +30,7 @@ header {
     position: fixed;
     top: 0;
     left: 0;
-    height: 50px;
+    min-height: 50px;
     width: 100%;
     border-bottom: 2px solid rgb(15, 38, 61);
     border-bottom: 2px solid rgba(15, 38, 61, 0.5);
@@ -59,11 +59,12 @@ header {
 
     .picture {
         float: left;
-        margin: 10px;
+        margin: 10px 10px 0 0;
     }
     .login {
         float: left;
         margin-top: 10px;
+        max-width: 208px;
         img {
             margin-top: 3px;
         }