We need to add activity statements for joining groups. Currently it just shows `X and Y performed the action "join"`.

An if statement should be added in the activity template and the appropriate keys should be added. We also need to make sure that the people joining the group are the preview part of the activity instead of the group itself, which means that the following code in the activity widget:

```
{if previewObj['oae:id'] === oae.data.me.id}
    {var previewObj = activity.object}
{/if}
```

will need to become

```
{if previewObj['oae:id'] === oae.data.me.id || activity['oae:activityType'] === 'group-join'}
    {var previewObj = activity.object}
{/if}
```