streams: Hide fake emails in stream creation form.

We hide fake emails in "People to add" section of
stream creation form and instead show user id
along with specifying that email is hidden.

tools/lib/capitalization.py

@@ -165,6 +165,8 @@ IGNORED_PHRASES = [
     # Used in our case studies
     r"Technical University of Munich",
     r"University of California San Diego",
+    # Used in stream creation form
+    r"email hidden",
 ]
 
 # Sort regexes in descending order of their lengths. As a result, the