VExistingUname: strip surrounding spaces

This was causing all pages that use this validator to be unable to
handle leading/trailing spaces around the username, since when chkuser()
is called it will always return None due to their presence. Changing
this will make a lot of pages more tolerant with usernames, like the
message compose page, pages for banning users, adding them as approved
submitters or moderators, etc.

r2/r2/lib/validator/validator.py

@@ -1596,6 +1596,8 @@ class VExistingUname(VRequired):
         VRequired.__init__(self, item, errors.NO_USER, *a, **kw)
 
     def run(self, name):
+        if name:
+            name = name.strip()
         if name and name.startswith('~') and c.user_is_admin:
             try:
                 user_id = int(name[1:])