Add posibility to check if plugin is installed and enabled from template

## Check if plugin is installed and enabled from template.

In Our current newscoop themes we have this problem that we want to show all our features, also those provided by available plugins. So to run theme we need to setup first all required plugins. This takes time and can create issues for new users.

From 4.3 ([PR-800][1]) we can use special function in smarty templates to check if plugin is available and enable all cool features in theme. 

Use example:

```
{{ if $gimme->isPluginEnabled('newscoop/community-ticker-plugin') }}
   // show community ticker
{{ /if }}
```

[1]: https://github.com/sourcefabric/Newscoop/pull/800