Allow groups to be used as aliases for user mentions

As discussed here: http://meta.discourse.org/t/give-groups-full-names-and-allow-mentions/11168/3 : 

Allows admins to configure whether a group name can be used as an alias for users. If so (and depending on the access level), the user can then use the @-mention search feature for new topics and in private messages to select a group, which then gets replaced by the lists of members to mentioned directly.

The default is to be set that no one can use a group as an alias.

Screenshots: it works the same way the usual mentions work, just that they are shown on the bottom after a break and without any image (you can see here in the overview it shows up to 3 usernames and the additional as a counter):

![screen shot 2014-01-04 at 21 37 41](https://f.cloud.github.com/assets/40496/1845321/1fd62668-7580-11e3-8f17-7f8f3b66c2c7.png)


Once you select that, it'll add the listed users to the conversation:

![screen shot 2014-01-03 at 17 57 26](https://f.cloud.github.com/assets/40496/1840796/8cf46892-7498-11e3-8e81-5b2ae07f0cd0.png)


The same for selecting users for private conversations:
![screen shot 2014-01-03 at 17 57 51](https://f.cloud.github.com/assets/40496/1840801/9fdeda1e-7498-11e3-8db2-1e3dbcb9ce70.png)

![screen shot 2014-01-03 at 17 58 03](https://f.cloud.github.com/assets/40496/1840805/a88c1910-7498-11e3-9b9c-6e21c1b31de9.png)

To enable this feature, the admin can set the alias-access-level per group in the admin backend UI:
![screen shot 2014-01-04 at 21 35 56](https://f.cloud.github.com/assets/40496/1845331/4d0a41b4-7580-11e3-9249-b166c5a9b72b.png)