Home: People invite UI fix

This branch fixes two issues:
It used to be that there were no limits on the number of people an user can invite. After this fix, the user be limited to inviting to up to 5 people. She/he will see the message "To keep the conversation close and meaningful, you can invite up to 5 people." displayed as the place holder text for the search box.
Question for @psybuzz, are users added based on the data-id that's in the DOM? I'm thinking of a senario when an user adds a person, but then removes the user from the selected list, is the person still added to the convo?

The second issue is that duplicate names are displayed when the user search for people. When a user selected a name, say 'Charles X', as one of the invitees, a subsequent search for 'Charles' will still show 'Charles X' in the dropdown of names. This push fixes that. Now the name is removed from the search results.
I'm also curious about the placeholder lines as well.  I think there's a line somewhere below that changes the `#people_input` placeholder to a different message, but it seems like the `.good_input` and `textarea` placeholder's don't really change, so we don't need to reset them.

But the `.val('')` makes sense.
Also, I'm wondering if we could be more specific than 'textarea'.  I know that we don't have any other textareas on the page, but if we add one in the future, we probably want this selector to just get the one inside the new-convo form.