Give feedback when adding existing project member

The members tab of project settings now gives better feedback when
trying to add a member to the project who already exists. Instead of
letting the "Add Existing" button be enabled (and changed to "Add New",
which would be completely misleading) it now disables the button if the
username typed into the field matches an existing user. (Either in name,
username or email address), and also displays a message explaining why
the button is disabled.

This commit also includes a test for this scenario.