FEATURE: add silence reason dropdown to admin penalty reason

Adds dropdown list for pre-defined penalty options to silence to
mirror options on suspension list.

app/assets/javascripts/admin/addon/components/admin-penalty-reason.hbs

@@ -18,15 +18,24 @@
       />
     {{/if}}
   {{else if (eq @penaltyType "silence")}}
-    <label class="silence-reason-title">{{html-safe
-        (i18n "admin.user.silence_reason_label")
-      }}</label>
-    <TextField
-      @value={{this.customReason}}
+    <label class="silence-reason-title">
+      {{html-safe (i18n "admin.user.silence_reason_label")}}</label>
+
+    <ComboBox
+      @content={{this.reasons}}
+      @value={{this.selectedReason}}
       @class="silence-reason"
-      @onChange={{this.setCustomReason}}
-      @placeholderKey="admin.user.silence_reason_placeholder"
+      @onChange={{this.setSelectedReason}}
     />
+
+    {{#if this.isCustomReason}}
+      <TextField
+        @value={{this.customReason}}
+        @class="silence-reason"
+        @onChange={{this.setCustomReason}}
+        @placeholderKey="admin.user.silence_reason_placeholder"
+      />
+    {{/if}}
   {{/if}}
 </div>