tweak add-ons search to search tags (bug 809590)

https://bugzilla.mozilla.org/show_bug.cgi?id=809590

**Steps to Reproduce**

- Create an add-on with an arbitrary tag (e.g. "randomtagxxx").
- In the add-ons search bar, search for "randomtagxxx".

**Expected Results:**

The add-on shows up as a result as it has image and zoom in its tags and description.

**Actual Results:**

The add-on does not show up.