DEV: Change settings root from plugins: to discourse_solved (#267)

This is so the plugins settings are better categorized in the site settings UI.

config/locales/client.en.yml

@@ -1,4 +1,9 @@
 en:
+  admin_js:
+    admin:
+      site_settings:
+        categories:
+          discourse_solved: "Discourse Solved"
   js:
     notifications:
       alt:

config/settings.yml

@@ -1,4 +1,4 @@
-plugins:
+discourse_solved:
   solved_enabled:
     default: true
     client: true