UX: Move solved filter behind plugin setting

On sites where solved is used but solved topics are not included in the knowledge explorer, having the filter caused issues.

assets/javascripts/discourse/controllers/knowledge-explorer.js.es6

@@ -75,7 +75,7 @@ export default Ember.Controller.extend({
 
   @discourseComputed
   canFilterSolved() {
-    return this.siteSettings.solved_enabled;
+    return this.siteSettings.solved_enabled && this.siteSettings.knowledge_explorer_add_solved_filter;
   },
 
   @discourseComputed("filterTags")

config/locales/server.en.yml

@@ -3,3 +3,4 @@ en:
     knowledge_explorer_enabled: "Enable the Knowledge Explorer Plugin"
     knowledge_explorer_categories: "A list of category slugs to include in the knowledge explorer"
     knowledge_explorer_tags: "A list of tags to include in the knowledge explorer"
+    knowledge_explorer_add_solved_filter: "Adds a filter for solved topics -- requires Discourse Solved to be installed and enabled"

config/settings.yml

@@ -10,3 +10,6 @@ plugins:
     type: tag_list
     default: ""
     client: true
+  knowledge_explorer_add_solved_filter:
+    default: false
+    client: true