add `Webapp.is_offline` and `?offline=True|False` to search API (bug 937698)

https://bugzilla.mozilla.org/show_bug.cgi?id=937698

<dl>
<dt><code>/api/v1/fireplace/search/?offline=True</code></dt>
<dd>show apps offering graceful offline support</dd>
<dt><code>/api/v1/fireplace/search/?offline=False</code></dt>
<dd>show apps requiring online connection</dd>
<dt><code>/api/v1/fireplace/search/</code></dt>
<dd>show all apps (regardless of offline/online support)</dd>
</dl>

PotatoSearch™ Fireplace patch to land after this bad boy gets merged in.