Don't bill (subtract gems) multiple times for multiple unlock item set calls (#12116)

* Issue 11050 - Small tuneups to unlock.js

- Use includes i.o. indexOf
- Extract small function for object setting duplication
- Use every instead of custom counter

* Issue 11050 - Properly store purchased items when purchasing them

* Issue 11050 - Couple more tuneups in unlock.js and implemented partial failure scenario

* Issue 11050 - Fix last lint issue

* Issue 11050 - Check path for gear i.o. failing to write it to purchased

* Issue 11050 - Guarantee variation coverage in tests

* Issue 11050 - Use startsWith instead of includes for background check

* Issue 11050 - Don't unlock lost items

website/client/src/pages/private-messages.vue

@@ -72,8 +72,8 @@
           />
         </div>
         <button
-          class="btn btn-secondary"
           v-if="canLoadMoreConversations"
+          class="btn btn-secondary"
           @click="loadConversations()"
         >
           {{ $t('loadMore') }}