CLC-4230 - Accept either UID or SID in the View As field

https://jira.ets.berkeley.edu/jira/browse/CLC-4230
Previous unmerged PR was #3058.

Only UIDs are stored in localStorage, but the full LDAP user objects will be accessible from memory, so there will still be access to SID, first name, etc. This should suffice until we get a back-end implementation.

![image](https://cloud.githubusercontent.com/assets/1585925/4673264/fa852426-559f-11e4-9f76-d7ae65ae8352.png)

Please write new tests for this model -- they should go in search_users_spec.rb.