adding search feature to users index page

Adding the search feature to the index page. User will now be able to go into the admin users page and do a query that will try to find any users that match. or partially match, on the name or email field.

/cc @zendesk/samson @pswadi-zendesk 

### References
 - Jira link: [Samson-94](https://zendesk.atlassian.net/browse/SAMSON-94)

### Risks
 - None
This test is brittle. We should check `assigns(:users).size.must_equal User.count`