Consider User trust level while showing Invites tab

Currently the Invites link and Invites tab content does not take User Trust Level into account. This PR does following changes:

* Trust Level 0 and 1 users will not see Invites link, and even if they try to access Invites section with URL, the content will not be shown.

![screen shot 2014-06-15 at 00 21 15](https://cloud.githubusercontent.com/assets/5732281/3279315/852395c0-f3f5-11e3-94bc-728bc451a316.png)

* Trust Level 2 and above will see Invites link and content, but they will not see extended message for Bulk Invite feature.

![screen shot 2014-06-15 at 00 09 06](https://cloud.githubusercontent.com/assets/5732281/3279313/851b1828-f3f5-11e3-8703-fb4fecda20cd.png)

* Only Admins will be able to see extended message for Bulk Invite feature.

![screen shot 2014-06-15 at 00 09 53](https://cloud.githubusercontent.com/assets/5732281/3279314/851f3eb2-f3f5-11e3-9e2f-e621a692453d.png)

cc @coding-horror, @nlalonde