FEATURE: Bulk Invite

This PR is for **[Bulk Invite](https://meta.discourse.org/t/bulk-invite-users/15899)** feature.

The *Batch Invite from File* button will only be visible to Admin on Invites page.

![invite](https://cloud.githubusercontent.com/assets/5732281/3185935/05e5176e-ec9a-11e3-841c-f75cc0be981a.png)

Once the user uploads the CSV file, the processing will happen in background thread.

If all the invites are sent successfully, user will receive a personal message notification, similar to as shown below:

![invite_success](https://cloud.githubusercontent.com/assets/5732281/3185934/05e03b40-ec9a-11e3-817b-07205054e6c0.png)

If some error occurs in processing, user will receive a personal message notification. It will show the exact error along with line numbers, similar to as shown below:

![invite_error](https://cloud.githubusercontent.com/assets/5732281/3185933/059a60de-ec9a-11e3-9b65-e95aefcec791.png)

**TO-DO**

* User need to be educated about file format. Only CSV file is allowed, and the email address needs to be in first column.
* Pre-staging the username (short+unique) and name (long+not-unique).
* Support Invitation to group.