Fixed one typo in response_helpers which caused is_admin context variable not to work correctly. Created new checkLinkNameForId function that checks if link_name is correct when creating new user and modifying existing one. This is now used for validation instead of doesLinkNameBelongToId in UserForm clean_link_name function. The previous validation function didn't allow to create new users and modify linkname of existing ones. Additionally I created new doesLinkNameExist helper function.

# Please sort this file in alphabetical order by last (given) name.
# Please only include one name per line.  If an (optional) email address is
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"Todd Larsen" <tlarsen@google.com>
"Sverre Rabbelier" <sverre@rabbelier.nl>
"Pawel Solyga" <pawel.solyga@gmail.com>