Introduces select-kit

* renames `select-box-kit` into `select-kit`
* introduces `single-select` and `multi-select` as base components
* introduces {{search-advanced-category-chooser}} as a better component for selecting category in advanced search
* improves events handling in select-kit
* recreates color selection inputs using {{multi-select}} and a custom {{selected-color}} component
* replaces category-selector by a component using select-kit and based on multi-select
* improves positioning of wrapper
* removes the need for offscreen, and instead use `select-kit-header` as a base focus point for all select-kit based components
* introduces a formal plugin api for select-kit based components
* introduces a formal pattern for loading and updating select-kit based components:

```
computeValue()
computeContent()
mutateValue()
```

app/assets/stylesheets/common/admin/admin_base.scss

@@ -212,10 +212,10 @@ $mobile-breakpoint: 700px;
 .admin-container {
   margin-top: 20px;
 
-  .select-box-kit {
+  .select-kit {
     width: 350px;
   }
-  .select-box-kit.multi-combo-box {
+  .select-kit.multi-select {
     width: 500px;
   }
   .select-box-kit.dropdown-select-box {
@@ -452,7 +452,7 @@ $mobile-breakpoint: 700px;
         width: 100%;
         padding-right: 0;
       }
-      .select-box-kit {
+      .select-kit {
         width: 100% !important; // Needs !important to override hard-coded value
         @media (max-width: $mobile-breakpoint) {
           width: 100% !important; // !important overrides hard-coded mobile width of 68px
@@ -623,7 +623,7 @@ section.details {
       margin-left: 0;
     }
 
-    .select-box-kit {
+    .select-kit {
       width: inherit;
     }
   }

app/assets/stylesheets/common/admin/flagging.scss

@@ -154,9 +154,9 @@
     align-items: center;
     justify-content: flex-start;
 
-    button, .select-box-kit {
-      margin-bottom: 0.5em;
+    button {
       margin-right: 0.5em;
+      margin-bottom: 0.5em;
     }
   }
 }