Add frontend for adding a category with the category picker

public/css/nimforum.scss

@@ -107,6 +107,24 @@ $logo-height: $navbar-height - 20px;
   }
 }
 
+#category-selection {
+  .dropdown {
+    .btn {
+      margin-right: 0px;
+    }
+  }
+  .plus-btn {
+    margin-right: 0px;
+    i {
+      margin-right: 0px;
+    }
+  }
+}
+
+.category {
+  white-space: nowrap;
+}
+
 #new-thread {
   .modal-container .modal-body {
     max-height: none;
@@ -715,12 +733,4 @@ hr {
 // - Hide features that have not been implemented yet.
 #main-buttons > section.navbar-section:nth-child(1) {
   display: none;
-}
-
-#threads-list.table {
-  tr > th:nth-child(2), tr > td:nth-child(2) {
-  }
-}
-
-.category, div.flag-button {
-}
+}